From: Star Trek: The Original Series
A species of highly intelligent noncorporeal beings whose sensory systems differ radically from humanoid sensory systems. This makes the Medusans particularly well suited for space travel.
While noncorporeal, Medusans still have a physical form, which when viewed by humanoids is said to be so ugly, that it will cause madness in the viewer unless that humanoid has the protection of a visor. In contrast, for those with telepathic capabilities, the Medusan mind is said to be remarkably beautiful.