1. (noun) In traditional Irish/Scottish music circles, an informal gathering of musicians for the purposes of playing tunes, having a few drinks, and socializing. A related (but not identical) term would be "jam session."
2. (verb) To play traditional music with another musician.
1. I'm going to bring my fiddle to the session in the hopes of getting in some tunes.
2. When the Chieftains came to town, we all sessioned until 3 A.M.