Ciro Santilli @cirosantilli 37

A member of the underlying field of a vector space. E.g. in ${\mathbb{R}}^3$, the underlying field is $\mathbb{R}$, and a scalar is a member of $\mathbb{R}$, i.e. a real number.

When Ciro Santilli says set theory, he basically means. Zermelo-Fraenkel set theory.