An existential clause is a °clause construction in which an °indefinite nominal is said to be in some location.
The definition is from Haspelmath (2025: 18). The nominal that is said to be in some location can be called "existent" (another term found in the literature is "pivot"). ❡
Note that the location can be implicit (and inferred from the context), but that "pure existence" is not included in the definition here (it is called "tautotic"). ❡
Since existential clauses are not really about "existence" but rather about location, Creissels (2019) proposes the term "inverse-locational" (at least for a subset of existential clauses, those that have "special syntax"). See also Creissels et al. (2016: §4.4).
existence event (SEM) = a situation in which the existence of some entity is presented. Existential situations favor a thetic construal. Example: There are apples in the kitchen expresses the existence of the relevant set of apples. (Section 11.3.1)