Negative crowd reaction. Boos, jeers, thrown drinks. Good heat means the heel is doing their job — the crowd hates the character. Bad heat (also called 'go-away heat' or 'X-Pac heat') means the crowd genuinely doesn't want to see you on their screen. The distinction matters enormously and management doesn't always read it correctly.
"He's got nuclear heat right now. People are booing before his music even finishes playing."