A semester or term was a time period during the study at a school.
Tom Paris studied two semesters biochemistry at Starfleet Academy. (VOY: "Parallax") During his second semester he chose the Starfleet base in Marseille and spent most of his time at Chez SandrÃne. (VOY: "The Cloud")
The Academy Extension Course was a semester-long course at Starfleet Academy. (VOY: "Non Sequitur")
Chakotay dodged asteroids for a semester during his time at Starfleet Academy. (VOY: "Future's End")
A semester of Interspecies Protocol was a necessity at Starfleet Academy. (VOY: "The Disease")
Kathryn Janeway was the guest lecturer in Reginald Barclay's semester course about the Borg at Starfleet Academy. (VOY: "Endgame")
In 2370, Beverly Crusher feared her son Wesley Crusher would wash out of Starfleet Academy next term if his grades did not improve. (TNG: "Journey's End")