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Ensign is the most junior commissioned officer rank, subordinate to a Lieutenant JG. It has a ranking code of O-1 and is equivalent to a Second Lieutenant in the Starfleet Marine Corps. The rank insignia consists of one full pip.
Ensigns are the most inexperienced officers in Starfleet, and as such need to have proven themselves to have exceptional skills in the Academy in order to become a Department Head and/or a bridge officer. As an officer however, they are still the superior officer of all non-commissioned officers and enlisted personnel.