Frottage
Frottage or frot is a type of non-penetrating sex between men. They rub each other with erect penises. Partners usually face each other. Frot provides many pleasant sensations, because the members of both partners are stimulated at the same time, especially the glans
Essentially the same activity between females is called tribadism.
In other species
Frottage is not limited to humans, and also occurs in other mammals. It has been observed with bonobos, manatees and bottlenose dolphins.
References
Sexual acts