A biquadratic extension of a field $F$ is a Galois extension $K$ of $F$ such that $\operatorname{Gal}(K/F)$ is isomorphic to the Klein 4-group. It receives its name from the fact that any such $K$ is the compositum of two distinct quadratic extensions of $F$. The name can be somewhat misleading, however, since biquadratic extensions of $F$ have exactly three distinct subfields that are quadratic extensions of $F$. This is easily seen to be true by the fact that the Klein 4-group has exactly three distinct subgroups of order (http://planetmath.org/OrderGroup) 2.
Note that, if $\alpha,\beta\in F$, then $F(\sqrt{\alpha},\sqrt{\beta})$ is a biquadratic extension of $F$ if and only if none of $\alpha$, $\beta$, and $\alpha\beta$ are squares in $F$.