A Line with a fixed scale so that every Real Number corresponds to a unique Point on the Line. The generalization of the real line to 2-D is called the Complex Plane.
See also Abscissa, Complex Plane
Courant, R. and Robbins, H. What is Mathematics?: An Elementary Approach to Ideas and Methods, 2nd ed.
Oxford, England: Oxford University Press, p. 57, 1996.