Please avoid using any colloquialism or slang in any of your formal essays.
Using a colloquialism can be a good way to add some everyday language to a piece of creative writing.
Colloquialism: The machine has to learn how to recognize and translate colloquial phrases as well as idiomatic expressions such as C'est quoi, ça?
The Pelican has used an English colloquialism which has a similar meaning.
The privatized rail system has been such a disaster that Railtrack has become a colloquialism for a chaotic and crazy way to run things.