Related terms include cisgender, cisnormativity, ally, and cissexism.
"Karen is a dedicated ally-- she's a cishet but goes to Pride every year to show support for her dads."
Cishet, used as both an adjective and a noun, describes a person who is both cisgender and heterosexual. A person is cishet if he or she is cisgender, meaning identifying with his or her assigned-at-birth gender, as well as heterosexual, or attracted exclusively to people of the opposite sex. Cishet individuals are not inherently part of the LGBT+ or queer community but may identify themselves as allies.
People who are cishet arguably form most of the human population. The common assumption that all or most people are cishet contributes to cisnormativity and heteronormativity. Most people who are cishet do not identify themselves as cishet because they have never had want or need of a specific label to describe their orientation and gender identity.
The use of "cishet" by the LGBT+ and feminist community has been a source of some controversy, It may be occasionally used in what are perceived as ad hominem or straw-man arguments when addressing cisgender heterosexuals, and this has led to many perceiving it as an insult. In gender, cishet is a shorthand descriptive term and not a slur.
First used in online communities in the 2000s, cishet is a portmanteau of cis-- as in cisgender, from the Latin cis-, meaning "this side of"-- and het-- as in heterosexual, meaning attracted to the opposite sex.