Interjection Definition
An interjection does not carry any grammatical meaning. It merely expresses a reaction of some kind. As such, adding an interjection to a phrase (or removing one) does not alter the phrase’s meaning.
Words such as oh, uh, wow, and yikes are interjections.
When to Use Interjections
Interjections in formal writing
Interjections should be avoided in formal writing, except if used in dialogue. For example, you should not write
Wow, I can’t believe how gorgeous it is!
However, it could be acceptable to write
When I saw how gorgeous it was, my reaction was to simply say, “Wow!”
Using interjections for impact
When using interjections is acceptable, they should be used sparingly. The emotional reaction that interjections are intended to convey is greatly diminished if they are overused.
For example, if someone wrote a quick blurb about a diet plan of which she is skeptical, and she wants to fully express her skepticism, she might use the interjection hmm as follows.
The creators of the diet claim that you can lose fifty pounds in one month. Hmm, I’m not too sure about that.
Notice how the author’s doubt is not quite as strong without the interjection:
The creators of the diet claim that you can lose fifty pounds in one month. I’m not too sure about that.
However, if the interjection does not make an impact of any kind, it is a good idea to leave it out.
Reference:
Aarts, B. (2011). Oxford modern English grammar. New York, NY: Oxford University Press.