The symbols most often used to denote a real variable are $x$ and $y$.
When it occurs in a mathematical equation, a variable is often referred to as an unknown.
In the specific context of elementary algebra, the ugly misnomer pronumeral is frequently found in Australia. This was introduced by extension of the concept of a pronoun: a symbol that stands in for a numeral, by which the term number is actually meant.
Thankfully the term appears not to have caught on in general.