I thought it might do to clarify what exactly a "jam session" is and is not, given recent overuse of the term, in the context of which it has perhaps lost its original usefulness.
Basic definition: a jam session is an informal gathering of musicians, or better yet of people playing music, who are not being paid to do so and are not required to do so for any external reason. They are playing for fun. Validation and status-seeking are not involved. Accruing of musical knowledge/experience is perhaps resultant but is not a primary goal of the proceedings, which occur entirely for the purpose of enjoyment.
Important distinction: a jam session is not a performance. Our concept of "performance" implies a strict separation between performers and spectators. In a jam session atmosphere there are ideally neither. Everyone is a participant, but none could be said to be "performers" since they aren't performing for anyone, not even really for each other. Frequently additional people are there listening and not making any music themselves; that's fine, but if these people are there in large part TO listen, you're dealing with a performance, not a jam session.
1) If it is scheduled in advance for some public location and advertised, regardless of what they call it, it is probably not a jam session.
2) If you care in any way whether you sound good, it is probably a performance, not a jam session.
3) If you are worried about people thinking you aren't good enough to be playing, it is not a jam session.
4) If the word "enough" crosses your mind at all, it is probably not a jam session. (Occurrence following the word "drunk" is acceptable.)
4) If some of the musicians involved are strangers, this is fine; if you care that they are strangers, it is probably not a jam session.
10 Best of 2014
January: Wyoming and the Open
February: New Mexico and the Holes
Notes on The Accounts