  Contrary to popular misconception, the question in the title is far from
simple. It involves sets of numbers on the first level, sets of sets of numbers
on the second level, and so on, endlessly. The infinite hierarchy of the levels
involved distinguishes the concept of "definable number" from such notions as
"natural number", "rational number", "algebraic number", "computable number"
etc. (Explanatory essay for non-experts.)
