To put a made thing into the place where it will actually run. Deployment is where a build meets reality.
Old French desploiier (to unfold), from Latin displicare (to scatter, unfold), from dis- (apart) + plicare (to fold); to put a made thing into the place where it will actually be unfolded and run, past all rehearsal.
The Old French root is an unfolding, not a placing — a deploy takes something folded up in preparation and opens it into the shape it will actually run in. Everything before that moment stays folded, however finished it looks.