Is substitute a noun

Yes, the Word 'substitute' is a noun, a singular, common, abstract noun; a Word for someone or something that takes the place of another. The Word 'substitute' is also a verb (substitute, substitutes, substituting, substituted). Examples:

Noun: Salsa is a good substitute for tomatoes in a sandwich.

Verb: Ms. Lincoln will substitute for Ms. Washington while she is on her honeymoon.