Interjections are words or phrases that express strong emotion. They stand alone and are not grammatically connected to the rest of the sentence.
Here's why "ouch" fits the definition:
* Expresses emotion: It clearly shows pain or surprise.
* Stands alone: You can use "ouch" by itself without needing to add other words.
* Not grammatically connected: It doesn't function as a noun, verb, adjective, etc., in a sentence.