preimage
noun
°The set containing exactly every member of the domain of a function such that the member is mapped by the function onto an element of a given subset of the codomain of the function. Formally, of a subset B of the codomain Y under a function ƒ, the subset of the domain X defined by.
"For example, the preimage of {4, 9} under the squaring function is the set {−3,−2,+2,+3}."
synonyms: inverse image