This is indeed a "technical" term in math. Wolfram has a good definition for what a "countably" infinite set is: http://mathworld.wolfram.com/CountablyInfinite.

This is indeed a "technical" term in math. Wolfram has a good definition for what a "countably" infinite set is: http://mathworld.wolfram.com/CountablyInfinite.html http://mathworld.wolfram.com/CountablyInfinite.html