That's TeX for "l" with subscript infinity. It's a vector norm, and is equal to the element of the vector with maximal absolute value. It's also called the ma

That's TeX for "l" with subscript infinity. It's a vector norm, and is equal to the element of the vector with maximal absolute value. It's also called the max-norm and sup-norm (for the infinite dimensional generalization, e.g., where the vector becomes a real-valued function of a single real variable).