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From Schmid.wiki
Background
The notion of set size as the existence of a bijection was invented (discovered?) by Georg Cantor (1845 - 1918), the creator of modern set theory. Cantor was the son of a Danish merchant, Georg Waldemar Cantor. See Wikipedia for more information.
Definition
Quote from Wikipedia:
When comparing two sets, we say that a set A and a set B have the same cardinality if and only if there exists a bijection, i.e. a one-to-one and onto function, between the two sets.
The same written in first order predicate logic:
