faithful steadfast in affection or allegiance; "years of faithful service"; "faithful employees"; "we do not doubt that England has a faithful patriot in the Lord Chancellor"
unfaithful not trustworthy; "an unfaithful reproduction"
quality an essential and distinguishing attribute of something or someone; "the quality of mercy is not strained"--Shakespeare
constancy the quality of being enduring and free from change or variation; "early mariners relied on the constancy of the trade winds"
Fidelity is the quality of being faithful or loyal. Its original meaning regarded duty to a lord or a king, in a broader sense than the related concept of fealty. Both derive from the Latin word fidēlis (A III adjective), meaning "faithful or loyal" In modern human relationships, the term can refer to sexual monogamy.
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fidelity Faithfulness to one's duties.
fidelity Accuracy, or exact correspondence to some given quality or fact.