Acting ensign is a provisional rank occasionally given to individuals at the discretion of their commanding officers.

In 2253 Captain Christopher Pike made cadet Spock an acting ensign when he accepted the role of science officer on the USS Enterprise. (EV comic: Flesh of My Flesh)

In 2364 Captain Jean-Luc Picard of the USS Enterprise-D rewarded the promising young Wesley Crusher with the rank of acting ensign. (TNG episode: Where No One Has Gone Before)