A strap is a straight length of leather, plastic or tearproof fabric, usually between half an inch and two and a half inches wide, similar to a belt, but usually shorter and designed for some other purpose than to be worn around the waist. In the context of BDSM, straps are a useful tool for bondage as they can be quickly tightened and secured and the broad width of the strap has less of a tendancy to cut off the circulation. The disadvantage of straps is that they can slide down limbs thus making the bondage less effective. Strapping is often used with various types of furniture to secure the victim.
The strap is also a popular tool for physical punishment; in this context, it is interchangeable with a belt.
Another type of strap used for bondage is the ratchet strap. Ratchet straps are made from webbing and use a small lever and ratchet to tighten the strap. They have the advantage over buckle type straps that they have an unlimited range of positions they can be tightened to, allowing for precise control of the tension of the strap. Ratchet straps come in a wide variety of colours and can be sourced without difficulty on the web.