Engaging in sexual relations only with one's spouse or long-term sexual partner.
Consistent with reality.
Reliable; worthy of trust.
Loyal; adhering firmly to person or cause.
Injective in specific contexts, e.g. of representations in representation or functors in category theory.
Having faith.
Someone or something that is faithful or reliable.
The practicing members of a religion or followers of a cause.
Being exclusively faithful to one partner; monogamous (especially as contrasted with polyamorous).