For the financial services company, see Fidelity Investments. Fidelity is also the name of a village in Illinois and a town in Missouri.

Fidelity is a notion, that at its most abstract level implies a truthful connection to a source. 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 fidelitas, meaning "faithfulness."

In modern human relationships, fidelity usually means strict adherence to marriage vows, or of promises of exclusivity or monogamy, and a lack of adultery.

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.