A fairly large frontal facing insturment which some people call a marching French Horn.
I like to define it as follows.
Mellophone, Noun - A trumpet(not capitalized) with a bigger penis. Used by those who either play Horn in symphony or have realized in all of their infinite wisdom that trumpets suck at life.
"Mellown" player: "I'm a Mellophone player, I'm out of tune always but I make up for it by playing at retarded volumes because dynamics don't matter in marching band, right?
Mark: "I'm a trumpet player and that makes me a horrible person, I'm stupid, annoying, and ugly, no one will ever love me genuinley, and pain and misery will haunt my pathetic life for all eternity"