adjective Concerning the very existence of something, especially with regard to evading extinction.
an existential risk or threat
adjective Of an assertion, etc.: assuming or suggesting the existence of something.
adjective Of or relating to existentialism (“a philosophical movement emphasizing the uniqueness of each human existence in freely making its self-defining choices”).
adjective Relating to part of a clause that indicates existence (for example, there is).
noun Ellipsis of existential clause (“a clause that indicates the existence of something”).
noun Ellipsis of existential type (“in a type system: a type that hides the underlying concrete type(s)”).