One of the elusive Holy Grails of rollercoaster fans,
airtime refers to a moment of negative G-force. The name is based on the motion of the rider during said moment. As the coaster train falls away from the rider (such as when cresting a hill,) the rider's body rises from the seat and floats ever so briefly. This brief moment of limited contact with the train does ever-so-much-fun things to the rider's internal organs.

Airtime is a nebulous thing, and sometimes takes a little looking for. Some coasters have it in abundance. Some don't have it at all. Seat selection can heavily influence airtime. The basic rule is that the closer the rider is to the front of the train, the more airtime he or she will experience. This is not an absolute, however, and will require experimentation with a given coaster.

Airtime can also be affected by weather, (how hot it is,) by time of day, (trains run faster as the day progresses,) and by how many people are riding. (Trains run faster with more riders.)

"Airtime" is also the name of the excellent newsletter published by the RollerCoaster Club of Great Britain.