Faithfulness is the concept of unfailingly remaining loyal to someone or something and putting that loyalty into consistent practice, regardless of extenuating circumstances. It may be applied to a husband or wife who, in a sexually exclusive marriage, does not engage in sexual relationships outside of the marriage, a customer at a restaurant who regularly dines there, or even to God himself with regard to his perpetual love towards his children that is not dependent on their worthiness. It could also mean keeping to one's promises no matter the prevailing circumstances. Literally, it is the state of being full of faith in the somewhat archaic 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, whereas a spouse who, inside a sexually exclusive relationship, has sexual relations outside of marriage could equally be considered as being "unfaithful" as having committed "infidelity".