frightening vs terrible

frightening is a noun but terrible is not a noun.

Word	Noun	Adjective	Verb	Adverb
frightening	Yes	Yes	No	No
terrible	No	Yes	No	No

As adjectives, frightening and terrible are synonyms defined as:

Other synonyms of frightening include awful, dire, direful, dread, dreaded, dreadful, fearful, fearsome, horrendous, horrific.

frightening (noun)	terrible (noun)
the act of inspiring with fear

frightening (adjective)	terrible (adjective)
causing fear or dread or terror	causing fear or dread or terror
	extreme in degree or extent or amount or impact
	intensely or extremely bad or unpleasant in degree or quality
	exceptionally bad or displeasing