A Property Holding Almost Everywhere

Let $(X,\mathcal{F},\mu)$ be a measure space and let $Q$ be a property of points of $X$. Then $Q$ holds $\mu$-almost everywhere, abbreviated $\mu$-a.e., if the set of points of $X$ at which $Q$ fails is $\mu$-null. When the measure is understood, one writes almost everywhere and a.e.; for Lebesgue measure $\lambda_n$ on $\mathbb{R}^n$ one says that $Q$ holds almost everywhere on $\mathbb{R}^n$.