Cite as: 523 U. S. 382 (1998)
Opinion of the Court
tion of "reserve strengthening" to include any net additions to reserves is based on a permissible construction of the statute. 111 F. 3d 1056 (1997). (It expressly disagreed with the Eighth Circuit's conclusion in Western National Mutual Insurance Co. v. Commissioner, 65 F. 3d 90 (1995), that the Treasury Regulation is invalid.) We granted certiorari. 522 U. S. 931 (1997).
II
The 1986 Act does not define "reserve strengthening." Atlantic contends that the term has a plain meaning under the statute: reserve increases attributable to changes in methods or assumptions. If that is what the term plainly means, Atlantic must prevail, "for the court, as well as the agency, must give effect to the unambiguously expressed intent of Congress." Chevron U. S. A. Inc. v. Natural Resources Defense Council, Inc., 467 U. S. 837, 842-843 (1984).
Atlantic contends that the plain meaning of "reserve strengthening" can be discerned, first, from its use in the PC insurance industry. It presented at trial two expert reports which, by "constructing a working definition of the term" that requires "a material change in methodology and/or assumptions," App. 68, 74, purport to demonstrate that Atlantic "did not strengthen reserves," id., at 99. Our task, of course, is to determine not what the term ought to mean, but what it does mean. Atlantic's first expert, before "constructing" a definition, expressly acknowledged that "re-serve strengthening" is "not a well-defined PC insurance or actuarial term of art to be found in PC actuarial, accounting, or insurance regulatory literature." Id., at 60. On this point she was in agreement with the Commissioner's experts: "In the property-casualty industry the term 'reserve strengthening' has various meanings, rather than a single universal meaning," id., at 124. If the expert reports establish anything, it is that "reserve strengthening" does not have an established meaning in the PC insurance industry.
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