The term "jam session" seems a tad modern, doesn't it?
Consider this account written by F. B. Converse, and published in The Cadenza in 1901.
"...I think it was sometime in 1856-57, or about the time of the "border-war" troubles, a jolly party of us, starting from St. Louis, on pleasure bound, decided to take a run up the Missouri. It was a jolly party, bent on fun, and having a good time generally. Of course we had a banjo, and would resort to the "Texas," where we had our musical seances.
"Well I do remember our landing in Kansas City. We were holding one of our seances as we approached the landing, or rather the levee..."
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I therefore submit for consideration that when we have a spontaneous, un-rehearsed, un-structured session of music at the upcoming AEBG, it should forthwith be referred to as a "seance."