adjective
Definition: Referring to something that existed or happened in the past but is no longer the case.
Example: The once bustling town is now quiet and empty.
adverb
Definition: At a single time; one time only.
Example: I visited that museum once when I was a child.
conjunction
Definition: At the time that something happens; as soon as something occurs.
Example: Once the sun sets, it gets dark quickly.
noun
Definition: A single occurrence or event; one time.
Example: I only visited that place once during my vacation.