What part of speech is at once?

"At once" can function as two different parts of speech depending on its usage:

* Adverb: When used to indicate simultaneity or immediacy, "at once" acts as an adverb.

* Example: "Please do it at once." (meaning "immediately")

* Prepositional phrase: When used to indicate a specific point in time, "at once" functions as a prepositional phrase.

* Example: "I saw him at once." (meaning "at that particular moment")

So, the part of speech of "at once" is determined by its context within the sentence.