A symbol for a value we don't know yet. It is usually a letter like x or y.
Example: in x + 2 = 6, x is the variable.
Why "variable" when it may have just one value? In the case of x + 2 = 6 we can solve it to find that x = 4. But in something like y = x + 2 (a linear equation) x can have many values. In general it is much easier to always call it a variable even though in some cases it is a single value.