Acting ensign
An acting ensign was when the rank of ensign was bestowed upon an individual in Starfleet to grant them access to locations and systems not normally accessible to civilians. 

Wesley Crusher was awarded a commission rank of acting ensign in 2364 by Captain Jean-Luc Picard in order to allow him access onto the bridge. In Picard's captain's log, he granted the rank "for outstanding performance in the best Starfleet tradition, Wesley Crusher is made acting Ensign with the duties and privileges of that rank and whether that rank becomes permanent, Mr. Crusher, depends on you." () 

After Crusher missed his opportunity to join Starfleet Academy in 2366, Picard told him that "in my eyes, you're an acting ensign in title only. I hereby grant you field promotion to full ensign with all the responsibilities and privileges of that rank." ()
Military ranks
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