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1.
A beer that is low in alcohol and has a balanced flavor of both hops and malt. The purpose being so that it can be drank over a long session of time with out overwhelming the palate or getting the drinker too intoxicated.
A light-bodied summer ale would make a good session beer.
by OregonBeerMan July 12, 2006
 
2.
A beer that goes well with marijuana smoking, or more specifically, a beer that is good to drink during a marijauna smoking session.
"Hey man, let's go pick up some Heinekens and Sierra Nevadas before the session, I'm gettin fuckin faded tonight!"

"Nah man, fuck that. Red Stripe is my favorite session beer!"
by JSnoop March 29, 2010
 
3.
A session beer is what a beer snob drinks when he wants to get plastered. Session beers are at least moderately expensive.
"Guinness is a good session beer, but I prefer Samuel Smith's Nut Brown Ale."
by neuroman December 20, 2005