The ability to record more than a single track on a disc. Track numbers are from 1 to 99. They continue to increment across session boundaries. e.g. if session 1 used tracks 1 to 4, session 2 would start at track 5. Track numbers may start at any value, but must be incremented sequentially on the disc.
Any recording device that can discretely record or re-record different "parallel" parts to separate tracks. In a studio, the multi-track recorder will typically handle 8, 12, 16, or 24 tracks, and multiple units can be synchronized together. (Thus studios may claim to have 48, 72, or more)