Frot
Frot or frotting (slang for frottage; from French frotter 'to rub') is a sexual practice that usually involves direct penis-to-penis contact. The term was popularized by gay male activists who disparaged or disliked the practice of anal sex, but has since spread in usage to others who may not share those attitudes. It can also be used as a type of foreplay.
Because it is not penetrative, frotting has the safe sex benefit of reducing the risk of transmitting HIV/AIDS; however, it still carries the risk of skin-to-skin sexually transmitted infections, such as HPV and pubic lice (crabs), both of which can be transmitted even when lesions are not visible.
It is analogous to tribadism, which is vulva-to-vulva contact.