An interjection is a word or short phrase that expresses strong emotion or a sudden reaction — surprise, joy, pain, relief, or hesitation — and is often followed by an exclamation point.
Examples: Wow! That’s amazing. Ouch! That really hurt. Well, I suppose we could try. Hey! Watch where you’re going.
Interjections stand apart from the grammar of the rest of the sentence — you can remove one without changing the sentence’s meaning. They’re there purely to add emotion, tone, and natural rhythm to speech and writing. Quick practice: pick out the interjection in “Yikes, I almost forgot my homework!” (Answer: Yikes.)