Colloquialisms are expressions that we use in casual or informal conversation. They are quite quirky, and vary according to your age, background, ethnic origin etc.

For example, you may call a carbonated soft drink pop, juice, ginger or soda, depending on where you’re from. In formal writing, you would need to call it a carbonated soft drink.

In formal and academic writing, colloquialisms such as:

'invite' instead of 'invitation'

'hooked up with' instead of 'met'

'a lot of' instead of 'a great many'

'is a really hot topic' instead of 'is causing current concern'

and other similar expressions should be avoided. The clue is that while you may see such expressions in the popular press, you won't find them in academic books or journals. At the university, you are part of a community of academics, and so, not unreasonably, you are expected to tailor your language to suit the academic environment.