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late 14c., "eternal, limitless," also "extremely great in number," from Old French infinit "endless, boundless," and directly from Latin infinitus "unbounded, unlimited," from in- "not, opposite of" (see in- (1)) + finitus "defining, definite," from finis "end" (see finish). The noun meaning "that which is infinite" is from 1580s.
1. Bigger than any natural number. There are various formal set definitions in set theory: a set X is infinite if
(i) There is a bijection between X and a proper subset of X.
(ii) There is an injection from the set N of natural numbers to X.
(iii) There is an injection from each natural number n to X.
These definitions are not necessarily equivalent unless we accept the Axiom of Choice.
2. The length of a line extended indefinitely.
See also infinite loop, infinite set.