Definition from Wiktionary, the free dictionary
- loyal; adhering firmly to person or cause
- My dog is a very faithful dog.
- having faith
- Some people are faithful to their god.
- reliable; worthy of trust
- My servant is very faithful.
- consistent with reality
- I would consider that a very faithful reproduction.
- engaging in sexual relations only with one's spouse or long-term sexual partner
- They had been faithful to each other all of their married life.
Derived terms 
loyal; adhering firmly to person or cause
reliable; worthy of trust
consistent with reality
See also 
- The practicing members of a religion or followers of a cause.
- The faithful pray five times a day.