Old Testament vs Pentateuch

Old Testament and Pentateuch both are nouns.

Word	Noun	Adjective	Verb	Adverb
Old Testament	Yes	No	No	No
Pentateuch	Yes	No	No	No

Old Testament (noun)	Pentateuch (noun)
the collection of books comprising the sacred scripture of the Hebrews and recording their history as the chosen people; the first half of the Christian Bible	the first of three divisions of the Hebrew Scriptures comprising the first five books of the Hebrew Bible considered as a unit