The interjection is a word or group of words which expresses strong emotions. These emotions may include sadness, joy, awe, sorrow etc. The interjection may also express an exclamation.
Here are some examples:
Oh my god!
As you can see from the above examples, the word is only an interjection when it is used as an interjection. This is also true for all other parts of speech: we only label a word after we see how it is used.
For example, in the above list, the word “watch” may also be used as a verb, and “out” as a preposition. However, in the expression “watch out!” those two words combine to form an interjection.