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Petitioners sought review of the IBIA decision pursuant to the Administrative Procedure Act, 5 U. S. C. §702. The District Court granted summary judgment in favor of the Secretary and other Department of Interior officials. As relevant here, the District Court determined that the plain language of 25 U. S. C. §479 defines “Indian” to include members of all tribes in existence in 1934, but does not require a tribe to have been federally recognized on that date. Carcieri v. Norton, 290 F. Supp. 2d 167, 179–181 (RI 2003). According to the District Court, because it is currently “federally-recognized” and “existed at the time of the enactment of the IRA,” the Narragansett Tribe “qualifies as an ‘Indian tribe’ within the meaning of §479.” Id.,at 181. As a result, “the secretary possesses authority under §465 to accept lands into trust for the benefit of the Narragansetts.” Ibid.

The Court of Appeals for the First Circuit affirmed, first in a panel decision, Carcieri v. Norton, 423 F. 3d 45 (2005), and then sitting en banc, 497 F. 3d 15 (CA1 2008). Although the Court of Appeals acknowledged that “[o]ne might have an initial instinct to read the word ‘now’ [in §479] . . . to mean the date of [the] enactment of the statute, June 18, 1934,” the court concluded that there was “ambiguity as to whether to view the term … as operating at the moment Congress enacted it or at the moment the Secretary invokes it.” Id.,at 26. The Court of Appeals noted that Congress has used the word “now” in other statutes to refer to the time of the statute’s application, not its enactment. Id.,at 26–27. The Court of Appeals also found that the particular statutory context of §479 did not clarify the meaning of “now.” On one hand, the Court of Appeals noted that another provision within the IRA, 25 U. S. C. §472, uses the term “now or hereafter,” which supports petitioners’ argument that “now,” by itself, does not refer to future events. But on the other hand, §479 contains the particular application date of “June 1, 1934,” suggesting that if Congress had wanted to refer to the date of enactment, it could have done so more specifically. 497 F. 3d, at 27. The Court of Appeals further reasoned that both interpretations of “now” are supported by reasonable policy explanations, id.,at 27–28, and it found that the legislative history failed to “clearly resolve the issue,” id., at 28.

Having found the statute ambiguous, the Court of Appeals applied the principles set forth in Chevron U. S. A. Inc. v. Natural Resources Defense Council, Inc., 467 U. S. 837, 843 (1984) , and deferred to the Secretary’s construction of the provision. 497 F. 3d, at 30. The court rejected petitioners’ arguments that the Secretary’s interpretation was an impermissible construction of the statute. Id.,at 30–34. It also held that petitioners had failed to demonstrate that the Secretary’s interpretation was inconsistent with earlier practices of the Department of Interior. Furthermore, the court determined that even if the interpretation were a departure from the Department’s prior practices, the decision should be affirmed based on the Secretary’s “reasoned explanation for his interpretation.” Id.,at 34.

We granted certiorari, 552 U. S. ___ (2008), and now reverse.

II

This case requires us to apply settled principles of statutory construction under which we must first determine whether the statutory text is plain and unambiguous. United States v. Gonzales, 520 U. S. 1, 4 (1997) . If it is, we must apply the statute according to its terms. See, e.g., Dodd v. United States, 545 U. S. 353, 359 (2005) ; Lamie v. United States Trustee, 540 U. S. 526, 534 (2004) ; Hartford Underwriters Ins. Co. v. Union Planters Bank, N. A., 530 U. S. 1, 6 (2000) ; Caminetti v. United States, 242 U. S. 470, 485 (1917) .

The Secretary may accept land into trust only for “the purpose of providing land for Indians.” 25 U. S. C. §465. “Indian” is defined by statute as follows:

“The term ‘Indian’ as used in this Act shall include all persons of Indian descent who are members of any recognized Indian tribe now under Federal jurisdiction, and all persons who are descendants of such members who were, on June 1, 1934, residing within the present boundaries of any Indian reservation, and shall further include all other persons of one-half or more Indian blood. . . . The term ‘tribe’ wherever used in this Act shall be construed to refer to any Indian tribe, organized band, pueblo, or the Indians residing on one reservation… .” §479 (emphasis added).

The parties are in agreement, as are we, that the Secretary’s authority to take the parcel in question into trust depends on whether the Narragansetts are members of a “recognized Indian Tribe now under Federal jurisdiction.” Ibid. That question, in turn, requires us to decide whether the word “now under Federal jurisdiction” refers to 1998, when the Secretary accepted the 31-acre parcel into trust, or 1934, when Congress enacted the IRA.

We begin with the ordinary meaning of the word “now,” as understood when the IRA was enacted. Director, Office of Workers’ Compensation Programs v. Greenwich Collieries, 512 U. S. 267, 272 (1994) ; Moskal v. United States, 498 U. S. 103, 108–109 (1990) . At that time, the primary definition of “now” was “[a]t the present time; at this moment; at the time of speaking.” Webster’s New International Dictionary 1671 (2d ed. 1934); see also Black’s Law Dictionary 1262 (3d ed. 1933) (defining “now” to mean “[a]t this time, or at the present moment” and noting that “ ‘[n]ow’ as used in a statute ordinarily refers to the date of its taking effect …” (emphasis added)). This definition is consistent with interpretations given to the word “now” by this Court, both before and after passage of the IRA, with respect to its use in other statutes. See, e.g., Franklin v. United States, 216 U. S. 559, 568–569 (1910) (interpreting a federal criminal statute to have “adopted such punishment as the laws of the State in which such place is situated now provide for the like offense” (citing United States v. Paul, 6Pet. 141 (1832) (internal quotation marks omitted))); Montana v. Kennedy, 366 U. S. 308, 310–311 (1961) (interpreting a statute granting citizenship status to foreign-born “children of persons who now are, or have been citizens of the United States” (internal quotation marks omitted; emphasis deleted)).

It also aligns with the natural reading of the word within the context of the IRA. For example, in the original version of 25 U. S. C. §465, which provided the same authority to the Secretary to accept land into trust for “the purpose of providing land for Indians,” Congress explicitly referred to current events, stating “[t]hat no part of such funds shall be used to acquire additional land outside of the exterior boundaries of [the] Navajo Indian Reservation . . . in the event that the proposed Navajo boundary extension measures now pending in Congress … become law.” IRA, §5, 48 Stat. 985 (emphasis added).4 In addition, elsewhere in the IRA, Congress expressly drew into the statute contemporaneous and future events by using the phrase “now or hereafter.” See 25 U. S. C. §468 (referring to “the geographic boundaries of any Indian reservation now existing or established hereafter”); §472 (referring to “Indians who may be appointed . . . to the various positions maintained, now or hereafter, by the Indian Office”). Congress’ use of the word “now” in this provision, without the accompanying phrase “or hereafter,” thus provides further textual support for the conclusion that the term refers solely to events contemporaneous with the Act’s enactment. See Barnhart v. Sigmon Coal Co., 534 U. S. 438, 452 (2002) (“[W]hen Congress includes particular language in one section of a statute but omits it in another section of the same Act, it is generally presumedthat Congress acts intentionally and purposely in the disparate inclusion or exclusion” (internal quotation marks omitted)).

Furthermore, the Secretary’s current interpretation is at odds with the Executive Branch’s construction of this provision at the time of enactment. In correspondence with those who would assist him in implementing the IRA, the Commissioner of Indian Affairs, John Collier, explained that:

“Section 19 of the Indian Reorganization Act of June 18, 1934 (48 Stat. L., 988), provides, in effect, that the term ‘Indian’ as used therein shall include—(1) all persons of Indian descent who are members of any recognized tribe that was under Federal jurisdiction at the date of the Act … .” Letter from John Collier, Commissioner, to Superintendents (Mar. 7, 1936), Lodging of Respondents (emphasis added).5

Thus, although we do not defer to Commissioner Collier’s interpretation of this unambiguous statute, see Estate of Cowart v. Nicklos Drilling Co., 505 U. S. 469, 476 (1992) , we agree with his conclusion that the word “now” in §479 limits the definition of “Indian,” and therefore limits the exercise of the Secretary’s trust authority under §465 to those members of tribes that were under federal jurisdiction at the time the IRA was enacted.

The Secretary makes two other arguments in support of his contention that the term “now” as used in §479 is ambiguous. We reject them both. First, the Secretary argues that although the “use of ‘now’ can refer to the time of enactment” in the abstract, “it can also refer to the time of the statute’s application.” Brief for Respondents 18. But the susceptibility of the word “now” to alternative meanings “does not render the word . . . whenever it is used, ambiguous,” particularly where “all but one of the meanings is ordinarily eliminated by context.” Deal v. United States, 508 U. S. 129, 131–132 (1993) . Here, the statutory context makes clear that “now” does not mean “now or hereafter” or “at the time of application.” Had Congress intended to legislate such a definition, it could have done so explicitly, as it did in §§468 and 472, or it could have omitted the word “now” altogether. Instead, Congress limited the statute by the word “now” and “we are obliged to give effect, if possible, to every word Congress used.” Reiter v. Sonotone Corp., 442 U. S. 330, 339 (1979) .

Second, the Secretary argues that §479 left a gap for the agency to fill by using the phrase “shallinclude” in its introductory clause. Brief for Respondents 26–27. The Secretary, in turn, claims to have permissibly filled that gap by defining “ ‘Tribe’ ” and “ ‘Individual Indian’ ” without reference to the date of the statute’s enactment. Id., at 28 (citing 25 CFR §§151.2(b), (c)(1) (2008)). But, as explained above, Congress left no gap in 25 U. S. C. §479 for the agency to fill. Rather, it explicitly and comprehensively defined the term by including only three discrete definitions: “[1] members of any recognized Indian tribe now under Federal jurisdiction, and [2] all persons who are descendants of such members who were, on June 1, 1934, residing within the present boundaries of any Indian reservation, and . . . [3] all other persons of one-half or more Indian blood.” Ibid. In other statutory provisions, Congress chose to expand the Secretary’s authority to particular Indian tribes not necessarily encompassed within the definitions of “Indian” set forth in §479.6 Had it understood the word “include” in §479 to encompass tribes other than those satisfying one of the three §479 definitions, Congress would have not needed to enact these additional statutory references to specific Tribes.

The Secretary and his amici also go beyond the statutory text to argue that Congress had no policy justification for limiting the Secretary’s trust authority to those tribes under federal jurisdiction in 1934, because the IRA was intended to strengthen Indian communities as a whole, regardless of their status in 1934. Petitioners counter that the main purpose of §465 was to reverse the loss of lands that Indians sustained under the General Allotment Act, see Atkinson Trading Co. v. Shirley, 532 U. S. 645 , n. 1 (2001), so the statute was limited to tribes under federal jurisdiction at that time because they were the tribes who lost their lands. We need not consider these competing policy views, because Congress’ use of the word “now” in §479 speaks for itself and “courts must presume that a legislature says in a statute what it means and means in a statute what it says there.” Connecticut Nat. Bank v. Germain, 503 U. S. 249, 253–254 (1992) .7
III
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