I was reading Undergraduate Algebraic Geometry by Miles Reid.
and on page 1. It says
If $k$ is in $\textbf{R}$ or $\textbf{C}$ (which it quite often is).
Now I should state some previous information.
A variety is (roughly) a locus defined by polynomial equation: $ V = \{P \in k^n | \text{f}_i(P) = 0\} \subset k^n $ where $k$ is a field and $\text{f}_i \in k[X_1, \cdots , X_n] $ are polynomials; so for example the plane curves $\textbf{C} : (\text{f}(x,y) = 0) \subset \textbf{R}^2$ or $\textbf{C}^2$
Now the question:
When are polynomials not in either $\textbf{R} \mbox{ or } \textbf{C}$??