\begin{definition}[Definition:Estimator]
Let $X_1, X_2, \ldots, X_n$ be random variables. 
Let the joint distribution of $X_1, X_2, \ldots, X_n$ be indexed by a population parameter $\theta$. 
Let $\delta: \R^n \to \R$ be a real-valued function. 
The random variable $\hat \theta = \map \delta {X_1, X_2, \ldots, X_n}$ is called an '''estimator''' of $\theta$. 
A particular realization of $\hat \theta$ is called an '''estimate''' of $\theta$.
\end{definition}