A cursive letter symbol commonly used to denote linear functions, line bundles, or angular momentum in mathematics and physics.
Examples
Index variable (common in sequences and series)
\sum_{\ell=1}^{n} a_{\ell}
Angular momentum in physics
\vec{\ell} = \vec{r} \times \vec{p}
Length function notation
\ell(w) = \text{length of word } w