Wiktionary
n. 1 A sexual activity in which two or more persons simultaneously masturbate themselves, usually while observing or otherwise communicating with one another. 2 A sexual activity in which two or more persons simultaneously masturbate one another.
Usage examples of "mutual masturbation".
If they went in for mutual masturbation, they'd likely need diagrams!
They had, she admitted to Anna, already gone as far as a sort of mutual masturbation, semi-clothed.
It had never ever gone further than gentle mutual masturbation, had lasted for over a year until Hugh graduated.
Much of their day passes in a sexual euphoria, with mutual masturbation and oral sex common among all members of the group.
The incident had ended in mutual masturbation, which, Pynchot supposed, he should have been grateful for - it was probably the only thing that caused them to really keep their promise.