Faithfulness means unfailingly remaining loyal to someone or something, and putting that loyalty into consistent practice regardless of extenuating circumstances. It may be exhibited, for example, by a husband or wife who does not engage in sexual relationships outside of the marriage.^[1] It can also mean keeping one's promises no matter the prevailing circumstances, such as in certain communities of monks who take a vow of silence.^[2] Literally, it is the state of being full of faith in the sense of steady devotion to a person, thing, or concept.

Its etymology is distantly related to that of fidelity; indeed, in modern electronic devices, a machine with high "fidelity" is considered "faithful" to its source material.^{[citation needed]} Similarly, a spouse who, inside a sexually exclusive relationship, has sexual relations outside of marriage could be considered as being "unfaithful" and as having committed "infidelity".^[3]

Sexual faithfulness within a marriage is a required tenet in Christianity—one of the four pillars of marriage.^[4] It is also required in Jewish marriage,^[5] and Islam.^[6]

• Fidelity – Quality of faithfulness or loyalty to another person or group
• Loyalty – Faithfulness or devotion to a person, country, group, or cause
• Marriage – Culturally recognised union between people