Fidelity is a notion that at its most abstract level implies a truthful connection to a source or sources. Its original meaning dealt with loyalty and attentiveness to one's 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. 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 designed for more accurate sound reproduction, while "lo-fi" music aims for "authenticity" over perfect production. Similarly in electronics, fidelity refers to the correspondence of the output signal to the input signal, rather than sound.

In the fields of scientific modelling and simulation, fidelity refers to the degree to which a model or simulation reproduces the state and behaviour of a real world object, feature or condition. Fidelity is therefore a measure of the realism of a model or simulation. Simulation fidelity has also been described in the past as 'degree of similarity'.

The computer age has spawned the term Wi-Fi in reference to certain groups of wireless electronic devices. While the term Wi-Fi has been popularly taken to be an abbreviation of 'wireless fidelity', Wi-Fi is in fact a commercial brand owned by the Wi-Fi Alliance, and has nothing to do with fidelity as a concept.