What is the common name for declaration, definition, expression and statement?

If established name doesn't exist, what 开发者_如何学Pythonname you can suggest?

You already seem to have an idea of what the standard names are. The four you've given are widely enough used that I'd go along with them unless you have some fairly specific reason to do otherwise. Of course, some depends on what "family" of languages you're talking about. Some terms are more widely used than others. Just for example, "special form" will probably be meaningful if your audience is users of Lisp and similar languages, but otherwise it might do more to obfuscate than clarify.

syntactic element? Not sure what you're getting at here...

Backus–Naur Form

If you're talking about things that might occur on the left hand side of a production in a formal grammar such as Backus-Naur form, I think the term you're looking for is nonterminal symbol.