Cishet, cishetallo, or monoallocishet is a term used by LGBTQ+ people to easily describe people who are not members of the community in any way, shape or form. This means people who are simultaneously cisgender, heterosexual, heteroromantic, allosexual, alloromantic, and monoamorous. The term originated within the trans community, particularly among straight trans people. It was used as a way to specify something who is not part of the LGBT+ community by virtue of sexuality or gender.
Exclusionists
Exclusionists have often used the term cishet to try to erase a-spec people (particularly heteroromantic asexuals and aromantic heterosexuals) from the community as they use the term to imply that a-specs aren't inherently LGBT+ by being a-spec. The term 'cishetallo' is preferable to make it explicit that one is not referring to a-specs.
Monoallocishet
There is debate over whether or not those under the polyamorous spectrum are inherently LGBTQ+. Those who consider polyam-spec individuals to be inherently LGBTQ+ may use the term monoallocishet to refer to all non-LGBTQ+ individuals. The 'mono' stands for monoamorous or monogamous, which is the act, capacity, desire, or preference of having only one partner.