Is only an adjective or an adverb?

"Only" can be an adjective or an adverb, depending on how it's used.

"You are the only one to succeed." "She is an only child." In these sentences, only is an adjective - it describes the nouns, rather than the verbs.

"It's only me!" Here, only is an adverb of degree, denoting something minor. "No, it's not a serial killer, it's only me."

"We only met yesterday." Here, only is an adverb of time, indicating that a minor span of time has elapsed.