The “computable” numbers may be described briefly as the real
numbers whose expressions as a decimal are calculable by finite means. Although the subject of this paper is ostensibly the computable numbers. it is almost equally easy to define and investigate computable functions of an integral variable or a real or computable variable, computable predicates, and so forth.

Source: http://www.cs.virginia.edu/~robins/Turing_Paper_1936.pdf