Wiktionary
free variable
n. 1 (context programming English) A variable that is not bind to a storage location. 2 (context mathematics logic English) A variable (occurring within some expression or well-formed formula) which is not bound by a quantifier or analogous symbol (such as a lambda abstractor, the "differential ''d''" near the end of an integral or differential form, or a summation symbol).
Usage examples of "free variable".
Instead, this one undulated like ripples on a pond, with time the apparent free variable.