Grinding is the act of two individuals rubbing hips together in a rhythmic motion. It is similar to dry humping, but is less sexual. It is often done on dance floors or other public or semi-public places. Typically, the term refers to erotic contact while both participants are fully clothed.
Grinding can also refer to masturbation performed in a similar way. In this case the participant is clothed and grinding against an object like a pillow or couch.
Although direct contact of the genitals never occurs, it is entirely possible to orgasm through grinding.