Why was "tantrum" trending last week?
A term describing a set which is isomorphic to a subet of the natural numbers. A countable set has "countably many" elements. If the isomorphism is stated explicitly then the set is called "a counted set" or "an enumeration".
Examples of countable sets are any finite set, the natural numbers, integers, and rational numbers. The real numbers and complex numbers are not [proof?].