One full trip of the button around the table (6 hands at a 6-handed table).
Everyone passes through every seat once, and your seat moves BB → SB → BTN → CO → HJ → UTG. Example: at a 6-handed table, the hands from one time you are the BB to the next are 6 hands. In mixed games, an orbit is also the unit for rotating games, and in Dealer's Choice each pick lasts one orbit starting with the chooser on the button.