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, meaning "faithful or loyal".

In modern human relationships, the term can refer to sexual monogamy. In western culture this often means adherence to marriage vows, or of promises of exclusivity or monogamy, and an absence of adultery. However, some people do not equate fidelity in personal relationships with sexual or emotional monogamy. (For example, see polyamory and Open marriage.) Often, however, females in Shakespeare are associated with it in a negative sense, such as "She is with little fidelity". For example, Bertram accuses Helena of having "little fidelity" in All's Well That Ends Well.

Fidelity also denotes how accurate a copy is to its source. For example, a worn gramophone record will have a lower fidelity than one in good condition, and a recording made by a low budget record company in the early 20th century is likely to have significantly less audio fidelity than a good modern recording. In the 1950s, the terms "high fidelity" or "hi-fi" were popularized for equipment and recordings which exhibited more accurate sound reproduction. The converse term "lo-fi", doesn't necessarily mean "low fidelity", rather that the production ethic aims for "gritty authenticity" over perfect production. Similarly in electronics, fidelity refers to the correspondence of the output signal to the input signal, rather than sound quality.