I'm reading about Jacobian Varieties... So I find the definition of theta divisor
$\Theta=\{[z] \in J(R)| \theta(\tau,z)=0\}$, where $\theta(\tau,z)$ is the theta funcion. Then the author writes:"The subvariety $\Theta$ is called a theta divisor."
My question is: why $\Theta$ is a subvariety? subvariety of $J(R)$? Any help is welcome, including bibliography. It's a new subject for me.
thank you!