To me a "gig" is a job, which means you get paid. You could participate in a "jam" at a club or friend's house. You could go to a "hootenanny" around a campfire. You can "woodshed" at home, or "play" a benefit. A gig is special. You're expected to set that date aside in your calender, show up and put on a professional musical entertainment, for hours specified, and expect payment in return. If anouther musician tells me that "we've" got a gig and wants me to participate, that's what I expect. If you want to call any of these examples a "gig", feel free, just don't change your terminology to your musical mates at the end of the evening in question. "Oh, when I said a "gig" I meant......."