What part of speech is proof?

The Word proof is both a noun and a verb.

Proof (as in evidence, or as in a final draft before printing of a photo or book) is a noun.

The corresponding verb is to prove.

Proof may be a verb, proof, meaning 'to test, to examine for flaws.'

In baking, it means 'to cause dough to rise.'

In printing, it means 'to make a trial impression' or 'to proofread.'

Examples:

Noun: For some people, if it's in a blog, that's proof that it's true.

Verb: More people should proof their emails and texts before they hit send.