Rubbing or grinding one's own crotch of one's own accord against an object (NOT another person), usually (but not always) for the purpose of masturbation. Does not apply to situations in which the crotch rubbing is unintentional or is being done by one character to another.
For rubbing against another person, use buttjob, frottage, grinding, penises touching, thigh sex, thigh straddling, or tribadism instead.