Adjective

(en adjective)

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.

Adjective

(en adjective)

Well grounded or justifiable, pertinent.

*{{quote-magazine, year=2012, month=March-April

, author=(Jan Sapp) , title=Race Finished , volume=100, issue=2, page=164 , magazine=(American Scientist) citation , passage=Few concepts are as emotionally charged as that of race. The word conjures up a mixture of associations—culture, ethnicity, genetics, subjugation, exclusion and persecution. But is the tragic history of efforts to define groups of people by race really a matter of the misuse of science, the abuse of a valid biological concept?}}

I will believe him as soon as he offers a valid answer.

Acceptable, proper or correct.

A valid format for the date is MM/DD/YY.

Do not drive without a valid license.

Related to the current topic, or presented within context, relevant.

(logic) Of a formula or system: such that it evaluates to true regardless of the input values.

(logic) Of an argument: whose conclusion is always true whenever its premises are true.

An argument is valid if and only if the set consisting of both (1) all of its premises and (2) the contradictory of its conclusion is inconsistent.