A long informal discussion usually amongst humans of the male variety. The word is most famously used in the Spongebob Squarepants episode"The Original Fry Cook." The word is used by Jim in his flashback. The topic of the discussion can be about anything. It is most common at mature teenage parties.
Guy 1: Dude me and my bros had a Bullsession the other night at Matt's party.
Guy 2: Dude thats awesome I wish I could have been there!