What is a 2255 Motion?

A definition from the United States Department of Justice Offices of the United States Attorneys website:

Federal prisoners may file two different kinds of motions for post-conviction relief: "Section 2255 motions" and "Section 2241 habeas corpus petitions."

Prisoners may file motions under 28 U.S.C. § 2255 challenging their convictions and sentences. A Section 2255 motion must be filed in the district where the prisoner was convicted and sentenced. The motion usually seeks to have the sentence or conviction vacated and may also request resentencing. As a general matter, Section 2255 is the proper vehicle for almost all federal prisoner collateral attacks.

[https://www.justice.gov/usam/usam-9-37000-federal-habeas-corpus]