Masturbation is the act of stimulating one's own erogenous zones (most notably the vulva/clitoris/vagina in females and the penis in males) with the aim of sexual self-gratification, whether alone or in the company of others. Masturbation need not conclude in orgasm.
Crotch rubbing is often used as a method of masturbation, especially by women. Other tools may be used (usually by women), such as vibrators, dildos, etc. Men sometimes use artificial vaginas or other sex toys, as well. Both sexes can also use their hands, alone.
source: danbooru wiki