Etymology
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infidelity (n.)

c. 1400, "want of faith, unbelief in religion; false belief, paganism;" also (early 15c.) "unfaithfulness or disloyalty to a person" (originally to a sovereign, by 16c. to a lover or spouse), from French infidélité (12c.) or directly from Latin infidelitatem (nominative infidelitas) "unfaithfulness, faithlessness," noun of quality from infidelis "unfaithful, unbelieving" (see infidel).

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Definitions of infidelity

infidelity (n.)
the quality of being unfaithful;
Synonyms: unfaithfulness
From wordnet.princeton.edu