Bob, what does "sitting" v. "standing" have to do with whether there's a PA?

Mr. Red, I agree that "sessions" implies a bunch of people playing and maybe singing along, pretty informally -- very different from an open mike. Probably not too much different from a "sing-along" except the emphasis is on instrumentals (or it may be entirely instrumental). The "jam sessions" I'm familiar with - folk, bluegrass, jazz -- usually involve different people taking short solo breaks too - again, informally.

"If you can sidle in and play what is being played it is a session." Yeah, that's what I thought. But I think many sessions are sort of intended to be informally restricted to people who actually can play without messing the others up. ;-D

Leadfingers, I'm not sure I get the point of "No matter how competent a performer may be, a James Taylor song in C Shapes capo One is NOT session material, but Sing Around/Open Mike stuff !"    Who tells people where to put their capo and what shapes to use as long as they're playing in the chosen key??   And those instructions wouldn't matter to the singers.

"If you want a Session , its where EVERYONE can join in !!"   That's always been my understanding. (If you're not very good, you just play softly enough that no one can hear you when you hit the wrong chord.)

As for the PA thing, the only reason I can think of for that being incompatible with a "session" is that there may be more players/singers than there are mics and amps. I know of some sessions where it's just a few people -- different people each week, and in someone's basement -- and they all use mics or DIs and amps.   That helps balance the sound out -- e.g., when you have the guitar doing battle with the accordion or a singer trying to be heard over a bass, 2 guitars, a piano, an accordion, 2 tambourines and a bodhron.   
There are also venues where the acoustics are such that amplification is really needed for some or all participants.
But, yeah, if it's one or two people up on the stage and amplified while the rest of the people sing or play along from their seats, that's not my idea of a "session."