An airport terminal is a building at an airport where passengers transfer between ground transportation and the facilities that allow them to board and disembark from aircraft.
Tag gates where passengers board or disembark from aircraft with aeroway=gate
Related facilities
Terminals, especially those at larger airports, often have shops and amenities (which can be tagged with shop=* and amenity=*).
They are often also form part of a greater public transport network and may have links to railways, buses and other modes of transport. Airports usually have ample parking available which can be tagged with amenity=parking.