I have stated my definition of deference as follows:
Deference is a court applying a reasonable agency interpretation of ambiguous statutory text despite the court's belief that there is a more reasonable interpretation of the ambiguous statutory text. That's it.
See
John A.
Townsend, The Report of the Death of the Interpretive Regulation Is an
Exaggeration 70 (SSRN last revised 12/15/21), here. This requires that the court actually determine
the best interpretation and then defer to a lesser agency interpretation
Some state that Chevron deference can include a court applying a reasonable agency interpretation without the court having to struggle with the issue of the best interpretation. I divide reasonable agency interpretations into two relevant categories: (i) the best interpretation, which is perforce reasonable; and (ii) an interpretation that is less than the best but still reasonable (whatever that means). Category (i) involves no deference; Category (ii) involves deference. If the court stops at determining that the agency interpretation is reasonable without determining the best interpretation, the court may or may not have deferred to a less persuasive agency interpretation.
These Categories and ones I discuss in my article Postscript (separately available here) depend upon the potential for a "best" interpretation. In Polselli v. United States Dep't of the Treasury-IRS. 23 F.4th 616 (6th Cir. 1/7/22), CA6 here and GS here, although not in a Chevron context, the dissenting judge stated the concept of the best interpretation as: "the least bad interpretation." For purposes of Chevron analysis, I think the best and the least bad interpretation are the same. The term "least bad interpretation" may be catchy, but I think the term best (or better) interpretation is more appropriate for statutory interpretation.
I discuss Polselli (although not in the Chevron context) in my earlier blog. 6th Circuit Holds Summonses in Aid of Collection Do Not Require Notice to Taxpayers Or Others (Federal Tax Procedure Blog 1/13/12, here); see also Leslie Book, Polselli v US: Circuit Split on Notice Rules For Summonses to Aid Collection (Procedurally Taxing 1/20/22), here.
Added 1/25/22 9:30am: