Not all reals are expressible: to be expressible is to have a finite representation in some language. Definable numbers [1] are that (countably infinite) subset

Not all reals are expressible: to be expressible is to have a finite representation in some language. Definable numbers [1] are that (countably infinite) subset of the reals which can be expressed individually. Almost all reals are not definable and therefore cannot be individually named. [1] https://en.wikipedia.org/wiki/Definable_real_number https://en.wikipedia.org/wiki/Definable_real_number