A sexual fetish
that involves being suspended or suspending someone.
It makes for great photography.
Suspension can be classified by body part used: people can be suspended in slings that hug their body, or they can be suspended from their extremities. The latter practice requires care, because the ropes/straps/webbing used can produce dangerous amounts of pressure on the limb.
Suspension can be a turn-on in itself, or it can be simply a part of more general immobilization, bondage and BDSM practices.
Suspension, particularly upside-down suspension, requires lot of communication and understanding between the people involved (dare I say suspensor and suspendee ? Nah), and should not last very long because it creates all sort of nasty stresses on muscle, legament and bones.
On the freakier, darker side, suspension should be contrasted with the hanging fetish.