double occupancy is the way in which almost all cruise fares and Tour packages are quoted, that is, based on two people traveling together.
Most Hotel rooms are quoted based on two adults to a room, as well
Other definition:
Double Occupancy is the way most hotel packages, tours and cruises are priced, based on two people staying together. Solo travellers usually pay a surcharge of up to 100\%, referred to as a ‘single supplement."
The Travel-Glossary: Everything about Travelling, Sightseeing and the World.