1. Infliction of severe physical pain as a means of punishment or coercion.
2. An instrument or a method for inflicting such pain.
3. Excruciating physical or mental pain; agony: the torture of waiting in suspense.
4. Something causing severe pain or anguish
5. Infliction of pain in order to attain sexual pleasures.
There is no exact definition of torture per-say. Each one of us followers of this painfully beautiful fetish defines it based on personal preferences.
Whereas bondage is rather soft, torture is meant to be hard. Hard-core if you will. Needles, suction, knives, plugs, scratches and bruises make some of us fly to the skies.