How to call something that can be either a terminal or a nonterminal?
The long forms "terminal symbol" and "nonterminal symbol" suggest that you are dealing first and foremost with "symbols"; "terminal" and "non-terminal" are qualifiying attributes that specify a symbol's role in the grammar. A symbol that can be either one of terminal and non-terminal is, almost by virtue of logics, just an arbitrary symbol.

Therefore, "symbol" seems to be the natural choice. I am pretty sure it is the term usually used, too, but such statements are hard to validate.

If you want to stress that a given symbol can be either kind (in text), you can always use "arbitrary symbol (of grammar $G$)", or define $X \subseteq t \cup T$ explicitly. In source code, I'd just use "symbol".