A statement made openly and on the record. A declaration binds the one declaring, which is what separates it from a remark.
Latin declaratio (a making clear, declaration), from declarare (to make fully clear), from de- (thoroughly) + clarare (to clarify), from clarus (clear, bright); a statement made openly enough to bind the one who makes it.
Latin clarus is clear, and a declaration's root promise is a clarity strong enough to be held to later. A remark can be walked back; a declaration is a statement made with that door already closed.