In professional wrestling, heat refers to both crowd reaction and real-life animosity between those involved in the professional wrestling business. In terms of crowd reaction, heat is usually either cheers for a babyface or boos for a heel. The amount of heat a wrestler generates is often an accurate gauge of his popularity.
Although the term can in some contexts refer to either positive or negative crowd reactions, "heat" can otherwise be used specifically to mean a negative crowd response (booing etc.); its opposite being a "pop" or positive reaction (cheering, clapping, etc.).
As heat typically refers to a negative reaction that a wrestling character gets from a crowd in a performance setting, it has also become slang for a negative reaction that a wrestler gets backstage from colleagues, management or both. Backstage heat can be garnered for both real and perceived slights and transgressions.
"Canned heat" refers to playing a recording of cheering or booing through the arena's sound system or adding it to a taped show. This serves to either amplify a crowd reaction or to mask silence from the crowd. Pre-taped crowd reactions from other events are also spliced in with the programming to make it look like the crowd is more energetic than it actually is, or if promoters want a storyline to go in a particular direction.
For example, in early 1992, the World Wrestling Federation was attempting to push Sid Justice as a heel. Toward the end of the 1992 Royal Rumble's main event, Justice – who had become popular due to his charisma – was loudly cheered when he eliminated Hulk Hogan; commentators Gorilla Monsoon and Bobby "The Brain" Heenan picked up on this as a fair act. However, the reaction was edited in future television replays, with Sid being booed heavily and Monsoon describing him as a jerk.
WWE overdubs cheers on pre-taped shows, particularly SmackDown. Professional wrestling magazine Power Slam joked that the company has had to "fire up the Fake Crowd Roar Machine™ to add an artificial atmosphere".
Heels draw "cheap heat" by blatantly insulting the fans, a local sports team, or the town in which they are performing. This is called "cheap" because it is an easy way for heels to receive boos. Faces would sometimes do the equivalent, referred to as a cheap pop, by referring to the town or promising to "win one for the fans". Mick Foley is well known for this using this particular form of getting a cheap pop.