To masturbate is to stimulate yourself sexually. In other words, to have sex by yourself, with yourself.
To masturbate means to have sex with yourself, usually using your hands. If you pick up a slang dictionary, you'll find hundreds and maybe thousands of terms that mean the same thing as masturbate. Those terms are almost all inappropriate, so use them carefully. Some religious traditions think that masturbating is a sin, but most people realize that there's nothing harmful or unusual about masturbating — it's a natural thing that humans do.
