\begin{definition}[Definition:Moment (Probability Theory)]
Let $X$ be a random variable on some probability space. 
Let $a$ be a real number.
Then the '''$n$th moment of $X$ about $a$''', usually denoted $\map {\mu_n} a$, is defined as: 
:$\map {\mu_n} a = \expect {\paren {X - a}^n}$
where $\expect X$ denotes the expectation of $X$.
\end{definition}