1. Noun. (mathematics) A number (real or complex) which is a solution to an equation of the form $x^2 + Bx + C = 0$ for some integers ''B'' and ''C''. For example 42, or 2+3''i'', or $1+\sqrt\left\{-3\right\} \over 2$. ¹

¹ Source: wiktionary.com