I see that for such a widely used term I cannot find any definition or dedicated wikipedia article. I also use the term but I'm not sure whether I use it correctly in all cases.
By looking at sites that describe language features (e.g. http://es6-features.org) I can have a sense of what they are, but without specific bounds.
I also see that features are usually categorized (like in the above site). But again I cannot find any site mentioning about categorization of programming language features.