Masturbation is the act of manually stimulating the genitals so as to achieve sexual pleasure, sexual arousal, and/or orgasm. Most often, masturbation is done by a person to himself or herself, but it is also possible to masturbate another person.
Masturbation and spanking
In some BDSM relationships, one partner (most often the submissive partner) may be forbidden from masturbating, or from doing so in the absence of the other partner, or from doing so without permission. Violations of such agreements may be reasons for spankings or other punishments.
(Male) masturbation is colloquially called spanking one's monkey.