(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)
In an alternate timeline created by Q, where Captain Jean-Luc Picard was given the chance to walk away from his fight with the Nausicaan that caused serious injury to his heart, which forced a bionic replacement to be installed, he returned to the present as a mere assistant astrophysics officer with the rank of lieutenant junior grade. It was later revealed by Q that it was that near-death experience that made the young Picard realize just how fragile life was and how important it could be, thus making him even more determined to make his mark in the universe and take risks, to earn his place as the captain of the USS Enterprise-D. (TNG: "Tapestry")
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||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|
|Services:||Andorian • Bajoran • Cardassian • Ferengi • Human (CSA • Third Reich • UE MACO • USA) • Jem'Hadar • Klingon • Romulan • United Earth/Federation Starfleet • 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".
Lieutenant junior grade is never seen on the 2140s-early 2160s uniform or referenced in Star Trek: Enterprise. A Star Trek: The Magazine article on the 2151 uniforms and rank did not mention lieutenant junior grade or lieutenant commander. (Star Trek: The Magazine Volume 3, Issue 11, p. 42)