A stylized calligraphic font variant commonly used for denoting categories, algebras, and special mathematical sets.
Examples
Category theory notation for a functor between categories
F: \mathcal{C} \rightarrow \mathcal{D}
Ring of linear operators on a vector space
T \in \mathcal{L}(V)
Probability space notation
(\Omega, \mathcal{F}, P) \text{ where } \mathcal{F} \text{ is a sigma-algebra}