(covers information from several alternate timelines)
"I couldn't say, though I doubt it."
The rank of lieutenant junior grade, sometimes abbreviated to lieutenant jg or junior lieutenant, was a junior officer grade used by Starfleet that was positioned between an ensign and a full lieutenant.
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. (Star Trek: The Motion Picture)
After taking the liberty to speak with Starfleet Medical, Captain Benjamin Sisko got them to waive the rest of Ezri Dax's training and gave her a commission as a full counselor with the rank of lieutenant. (DS9: "Afterimage")
Lieutenants junior grade
|Enlisted personnel||Commissioned officers||Generals and flag officers|
|ENS||LT JG||LT||LT CMDR||CMDR||CAPT||FCAPT||CDORE||RADM||VADM||ADM||FADM|
|2nd LT||1st LT||CAPT||MAJ||LT COL||COL||BRIG||MAJ GEN||LT GEN||GEN||MARSHAL|
|Positions:||Boatswain • Chief • Corpsman • Engineer's mate • Specialist • Technician • Yeoman||TAC • SO • 3rd • 2nd • 1st • CO • DD||Portmaster • Chief of Staff • CinC|
|Services:||Andorian • Bajoran • Cardassian • Ferengi • Jem'Hadar • Klingon • Romulan • SS • United Earth MACO • United Earth/Federation Starfleet • United States • Vulcan|
Mirror universe: Terran Empire Starfleet • Terran Empire MACO
The rank of "lieutenant junior grade" was portrayed in nearly all Starfleet rank schemes, but was only mentioned in the dialogue of the Star Trek: The Next Generation episode "Tapestry" and the Star Trek: Deep Space Nine episode "Prodigal Daughter", while the term "junior lieutenant" appeared in dialogue in "If Wishes Were Horses".
An issue of Star Trek: The Magazine(citation needed • edit) established that the costuming department on Star Trek: Enterprise had no insignia for lieutenant junior grade, although this does not necessarily mean the rank did not exist.