Acronym for Medicated Urethral System for Erection, a brand name. Abb. MUSE (MEWZ). Made by Vivus, it is a small pellet that is pushed into the tip of the penis with a small, specially-designed plunger. The pellet dissolves, and is absorbed by the tissues, to cause an erection. The active ingredient is alprostadil.
There is a section at Phoenix5 that gives more information.