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.