liar vs false witness

liar and false witness both are nouns.

Word	Noun	Adjective	Verb	Adverb
liar	Yes	No	No	No
false witness	Yes	No	No	No

As nouns, false witness is a hyponym of liar; that is, false witness is a word with a more specific, narrower meaning than liar:

Other hyponyms of liar include perjurer, fabricator, fibber, storyteller.

liar (noun)	false witness (noun)
a person who has lied or who lies repeatedly	a person who deliberately gives false testimony