Definition of orbit
What is meant by an orbit in poker is a complete turn of the table. It's the same vocabulary in tournament play as it is in cash games and sit 'n' go. The number of seats will not matter. Tournament players sometimes calculate their stack in rounds. For example, if you have a stack of 10,000 chips and the blinds are at 500/1000, you can say you have 10 big blinds, but you can also calculate that you will have roughly 6 orbits left before you are blinded (being blinded is having your stack eaten up by the blinds). Instead of saying that you have a stack of 10 big blinds, you would say that you have a stack that allows you to make 6 orbits.
Other details on the orbit
The orbit does not have to be a specific number of hands. Whether the table has 6, 9 or 10 players, when all the players have been once on each position, we speak of an orbit. On a poker table, each position has its own particularity (some are very powerful like the button position, others much less advantageous like the small blind position) and this is why the positions will change in turn. When each player has been in each position once, it is called an orbit.