"Heat" is a term used in
professional wrestling to describe any reaction to a wrestler and his gimmick/character.
Face and
heel heat can make or break a character, while not getting any heat is the
kiss of death in the industry.
Then there's
X-Pac Heat. This is when the audience boos and insults a wrestler not because they hate the character, but because
they hate the performer. It can be because they've been
pushed too hard, they have political power behind the scenes, or because they're terrible performers. For wrestlers, this often refers to guys who have poor matches (or actively wreck the pushes of others) and become overexposed despite crowds not caring for them one way or the other (either as heroes or as villains). This is not a heel, a villain whom fans
want to see punished; an X-Pac is someone fans don't want to see
at all. It's a very specific form of breaking
kayfabe by the fans and happens to be quite rare.
Note: "Not the right kind of heat" is an alternative meaning to "X-Pac Heat" that can, but does not always, intersect with it. It's more of a "we are sick of this character and don't want to see him anymore" reaction than a "we want this character to get his comeuppance" reaction (which is the impression you
want the fans to have). This is more or less the equivalent of a villainous
Scrappy, but has its own page.
Normally, it is possible to
correct an unfavorable response by altering a wrestler's character or shifting their position in the company. It is significantly harder to overcome X-Pac Heat; unlike
the Scrappy, the audience is reacting not to an annoying
character, but to a bad or overexposed
performer.
In short: if you
love to hate the character, it's heel; if you just hate the character, it's
the Scrappy; if you hate the performer, it's X-Pac Heat.
Note that the live audience in professional wrestling is considered to be
in-universe because it is such a major part of the show. When the reaction is obvious and on-camera, examples involving it can be considered objective, unlike those involving the viewers at home or other media.