A slang word developed and used by highschool marching band students to convey a sense of admiration, often bordering on sexual arousal. Induced either by an especially complex and well-executed piece of music or an exceptionally well-crafted musical instrument.
Bandmember A: Did you here the Blue Cavaliers' trumpet solo in the DCI championships?
Bandmember B: Oh man...horngasm
Bandmember A: *nods* Horngasm
An involuntary burst of magic from a unicorn's horn.
Twilight Sparkle had a huge horngasm when Rainbow Dash first did the Sonic Rainboom.