A person who is in your class who only associates with you in order to be more familiarized with the course and assignments.
Mandy invites Billy to her birthday party after finals but Billy declines due to the fact that classes are no longer in session.
Mandy sees Billy on campus next term. Billy walks by not acknowledging her hello. Billy was a semester friend.
Someone you hang out with for one, but up to two, semesters during college/university. After that, they drop off the face of the planet and/or make no effort to interact with you.
This state of "friendship" is further exacerbated if they're hostile or indifferent when you attempt contact with them.
Sam was just a semester friend. We hung out during Fall of 2008 and Spring of 2009, but now she never talks to me anymore.