In linguistics, a count noun (also countable noun) is a noun that can be modified by a numeral) and that occurs in both singular and plural forms, and that co-occurs with quantificational determiners like every, each, several, etc. A mass noun has none of these properties, because it cannot be modified by a numeral, cannot occur in plural, and cannot co-occur with quantificational determiners.
This is a companion discussion topic for the original entry at http://grammarerror.com/tags/countable-and-uncountable