Essays and theses/dissertations must be written in formal academic English, because informal writing involving the use of colloquialisms and slang language is frowned upon in the academic world.
In this context, universities, academic institutions and journal article publishers set a specific standard for the formal language used in academic manuscripts. Informal style of writing may be suitable in journalistic reports, novels or books, but not in a Master's or PhD thesis, journal article or other scientific or scholarly papers.
“We use formal language in situations that are serious or that involve people we don’t know well. Informal language is more commonly used in situations that are more relaxed and involve people we know well. Formal language is more common when we write; informal language is more common when we speak. However, there are times where writing can be very informal, for example, when writing postcards or letters to friends, emails or text messages. There are also examples where spoken English can be very formal, for example, in a speech or a lecture. Most uses of English are neutral; that is, they are neither formal nor informal. Formal language and informal language are associated with particular choices of grammar and vocabulary.
Contractions, relative clauses without a relative pronoun and ellipsis are more common in informal language.”
"You tend to find formal language in academic journals or official documents and notices where it brings an extra degree of seriousness to the subject. As a general rule, it isn't appropriate for everyday situations.
Here are some examples of formal words with their equivalents in standard English - notice that the formal words are often longer than the standard terms.It can be tempting to use formal vocabulary in the hope that it will add more weight to what you are saying, or just sound generally more impressive or sophisticated. You should generally try to resist this temptation. Using formal English in everyday situations can make your writing sound pompous or pretentious. You may also make what you've written sound unintentionally funny, as some writers deliberately choose formal vocabulary to create a comic effect."