Abbreviation for eXtreme Makeout Session.
A makeout session ranging from 1st to 3rd base depending on the situation, location, or mood.
Usually planned ahead with no emotional attachment or strings attached.
Lee: Hey Cat! Wanna have an xms later?
Cat: Heh, Lee and I had a crazzzyyyy xms last night at Kevin's party.
Amanda: ...is Lee available tonight?