Question

In the expression 6x, x is what?

Answer 1

`6x` in itself is a term...
Here... `x` is a variable that can hold any value... if it was a linear equation in one variable... we'd be able to find out the value

Answer 2

`x` is a variable

Explanation:

You could also hear "`x` is a variable quantity"

or, in some contexts, "`x` is an unknown" (or "an unknown quantity").