Lieutenant was a rank of a junior officer in most navy, land and marine forces. It was also used as a post in many navies. For instance, First Lieutenant was the most senior officer on a vessel after the commanding officer. On large ships, this post could be occupied by a Commander.
In the Republic of Cinnabar Navy, a midshipman had to pass Lieutanant's exams (boards), to become eligible for promotion to the rank. It was the lowest commissioned rank; thus the midshipmen were on the track to commission, but were not, in fact, commissioned officers.
See Ranks for comparisons of ranks in various armed services.