What is the accepted OG and ABV of a session beer? What other qualities does a session beer possess?
The definition varies, depending on who you ask (and usually upon their tolerance to alcohol and frequency with which they drink). But it means you should be able to have a couple and go back to work functioning perfectly well, or several over the course of a day without getting more than a little buzzed.
In my opinion, it should be less than 5%, but my weight is probably a little above average, so for others the 3%-4% range would probably be more appropriate.
Beer Advocate has a good article on the subject, including a little history: http://beeradvocate.com/articles/653
I don't know if there's an accepted standard or not, like "Must be under 4% ABV" or something. Personally, I define a session beer as something that is not too heavy in flavor or in my belly, that doesn't have too much alcohol. "Too much", for me, is defined as "I can drink this for many hours in the sun and still communicate with my parents."
I think it's subjective. But basically, it's a beer that you can drink, say, six or eight of over the course of three or four hours and still be able to function normally. For some people, PBR might be all they can handle in a session. For others, maybe it's a wee heavy. I deinitely want the guy who calls Samichlaus a session beer on my side!