When a band is forced to play a song in a lower key than it was originally recorded due to the singer's inability to reach the original key without making a mistake. It can apply to any band, whether they play original songs or covers. Named after ZP Theart, former lead singer of DragonForce, who made the band tune down between half a step and a full step because he had trouble singing the songs how they were originally made. This is not a necessarily negative thing as some songs are made interesting when in a lower key.
Me: Dude, you're downtuning the song 2 and a half steps?
Abe: Yep, what's wrong with that?
Me: Nothing at all... (walks away and does a facepalm) *whisper* darn Theart Syndrome...