A freshman was a first year student.
As a freshman cadet at Starfleet Academy, Jean-Luc Picard won the Starfleet Academy marathon, passing four upperclassmen on the last hill on the Danula II course. As of that year, he was the first freshman ever to win the event. Impressed by his feat, Admiral J.P. Hanson made a point of getting to know Picard. Hanson recalled Picard's victory in 2367 when stressing his belief that Picard would never help the Borg. (TNG: "The Best of Both Worlds, Part II")
In 2368, according to Captain Picard, ever since the earliest wormholes were discovered, first year philosophy students were asked the question "What if one of those lives I save down there is a child who grows up to be the next Adolf Hitler or Khan Singh?" (TNG: "A Matter Of Time")
In 2372, initially believing his fellow Academy cadets were dismissing him because he was a Ferengi, Nog realized it was more than that when he heard other freshmen complaining about similar treatment at the hands of upperclassmen. (DS9: "Homefront")