Ensign

Ensign is a military rank, the equivalent of which is used by the service organizations of many civilizations. As a traditional naval grade, Ensign is the most junior line officer rank below the Lieutenant grades, usually immediately under Lieutenant or Lieutenant Junior Grade. This is usually the first commisioned rank assigned to new officers (such as recent graduates of Starfleet Academy). This naval rank is equivalent to the rank of second Lieutenant which is found in the United States Army, Air Force, and Marine Corps; or the Romulan Star Empire's Sublieutenant grade.

An Ensign's primary duty is professional growth, both as a leader and as a occupational specialist. Ensigns are mentored by more senior officers as well as their (nominally) subordinate Chief Petty Officer as as part of their development as Starfleet leaders. As Ensigns prove themselves more capable, they are considered for promotion into the Lieutenant tier.

In the US Navy of old Earth, Ensigns wore a one-stripe insignia, which carried over to the 22nd century Earth Starfleet, which denoted Ensigns with a single rank pin. During intervening Federation Starfleet uniform schemes, Ensigns had no insignia at all until the 2280s when Ensigns wore a single pin shaped like a stylized "v" letter. In the 24th century uniform scheme of Starfleet, Ensigns were once again indicated by one single rank pin.

From Memory Alpha -- the free Star Trek reference