Is it correct because it appears in the dictionary?

A dictionary is descriptive – it describes the language as it is currently being used.

A style guide, on the other hand, is prescriptive – it describes how we should use language.

Style guides change much more slowly than dictionaries because new usages are always tried out and either accepted or rejected in the everyday language before making their way into standard usage.

The dictionaries show the usages being tried out; the style guides show the usages accepted as appropriate for formal English.

💡 For example, a dictionary will contain the word "ain't" because this word is used in everyday expressions. However, because this word has not been accepted as formal usage, a style guide will proscribe (prohibit) its use.

This text is an edited version of the following:

Witte F, American Medical Writers Association. Basic Grammar and Usage : An Essential Skills Workshop of the American Medical Writers Association. American Medical Writers Association; 2011: 63.