definition:
let be a relation and let be a variable, the expression means "there is an such that is true"
example:
consider the sentence "there exists a human that is polite" let a human be denoted by and a polite human by we can write this as or
the existential quantifier is an abbreviation for there is a.
(e.t. jaynes, 2003 chapter 1.8.2 nitpicking)