A slang term. "Leg" is a takeoff and landing. So if a crew takes off from KJFK, fly to KBOS, fly to KPWM, fly to CYUL and go off duty, that is three legs. Usually the captain and first officer will alternate being pilot flying and pilot monitoring from one leg to the next; they will say "that's the captain's leg" or "that's the first officer's leg".