Lieutenant (j.g.)
From Starbase 900 Wiki
Lieutenant junior grade often abbreviated to Lieutenant (j.g.), or junior lieutenant is a military rank, the equivalent of which is used by the service organizations of many civilizations. A naval rank in a small number of Earth's ancient navies (such as the US Navy), it is placed between Ensign and full Lieutenant. The rank is used primarily by Starfleet, although its United Earth predecessor, from which it primarily takes its rank structure, did not seem to have had this grade.
During certain points in its history, Starfleet discontinued the rank of Lieutenant junior grade, promoting Ensigns directly to the full grade of Lieutenant. This was the case in the mid 2270s, when Ensigns adopted the sleeve insignia of the former Lieutenant jg rank.
