A promise binding both sides, held by their own word rather than by force. A covenant survives because the parties keep it, not because it is enforced.
Old French covenant (an agreement), present participle of covenir (to agree, be fitting), from Latin convenire (to come together, agree), from com- (together) + venire (to come); a promise binding both sides by their own word, kept because they hold it rather than because anyone forces them to.
Latin venire is to come, and a covenant was first the coming-together itself, before it was the promise made once everyone had arrived. What holds it afterward is only the same willingness that brought the parties there in the first place.