I'm reading about curves and came across the statement that a curve $v: [0,1]\rightarrow S$ is "differentiable at $L^1$-a.e. point $t \in [0,1]$." $S$ is some metric space on which we're attempting to define differentiable curves.
I understand what it means for a property to hold a.e. (e.g. for $t\in [0,1]$ a.e. with respect to the Lebesgue measure) but not $L^1-a.e.$