What part of speech is the word instant?

The word "instant" can be two different parts of speech, depending on how it's used:

* Noun: When referring to a very short period of time, like "Wait just a moment, I'll be with you in an instant."

* Adjective: When describing something as happening or being done immediately, like "I need an instant solution to this problem."

So, the part of speech depends on the context of the sentence.