A thing another thing cannot work without. Every dependency is a place where someone else's choices reach you.
Latin dependere (to hang from, depend on), from de- (down, from) + pendere (to hang); a thing another thing cannot work without, named for hanging down from whatever it relies on.
It shares dependents' root of hanging from something else, but names the thing hung upon rather than the thing doing the hanging. Every choice made inside a dependency is a choice made, at a distance, for whatever relies on it.