Peter Masters(petermasters on FetLife)
Knife play is any BDSM activity primarily involving a knife. This can include cutting designs into the skin, and mind fucks.
As the skin may be broken during knifeplay, risk of infection needs to be a consideration.
Other BDSM activities which only secondarily involve knives (such as in bondage where they are used to quickly release a bottom or submissive), are not considered knife play.