The symbols most often used to denote a complex variable are $w$ and $z$.
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.
- 1981: Murray R. Spiegel: Theory and Problems of Complex Variables (SI ed.) ... (previous) ... (next): $1$: Complex Numbers: The Complex Number System
- 1981: Murray R. Spiegel: Theory and Problems of Complex Variables (SI ed.) ... (previous) ... (next): $2$: Functions, Limits and Continuity: Variables and Functions