ℝ

Etymology
In Stetigkeit und irrationale Zahlen (1872) uses R to denote rational numbers and ℜ to denote real numbers. The usage of blackboard bold dates to the 1960s.

Noun

 * 1)  The set of real numbers.
 * 2)  This set, equipped with any of its standard structure; the real line.

Derived terms

 * ℝ-order tree