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Definition of Preimage
1. Noun. (mathematics) 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 ¹
¹ Source: wiktionary.com