A camstrap is a piece of webbing with a buckle that is used to secure a cylinder or scuba tank to a buoyancy compensator or backplate. The name is derived from the cam

Cam

A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path...

action of the buckle to tighten and secure the strap. Also known as a cam band or tank strap. Cam straps are also typically used in whitewater rafting to tie rowing frames to the rafts. However, the cam buckle used in rafting cam straps is very different than one used in diving. The rafting cam strap has a self locking cam lever that cannot be accidentally releaseed. A thumb released cam locks the webbing in place and its locking action even allows tightening one handed.