An orbit in poker describes a full rotation of hands at the table, which means that every player at the table goes through the blinds once.
Regardless of when you start counting the orbit, it will end once the full rotation is over and the big blind comes back to the player that was in the blind when you started counting.
Thus, at a 9-handed table, one full orbit consists of nine hands. At a 6-max table, the orbit is six hands.
So, for example, if you hear a player say, I’ll play one more orbit before leaving, if they’re in the big blind, this means they’ll play until the big blind reaches them again, at which point they’ll leave.
Announcing “one more orbit” isn’t binding, though, so don’t be surprised if a player changes their mind and keeps on playing or leaves earlier.