mass noun

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A noun, such as sand, oil, or honesty, that denotes a substance or concept indivisible into countable units and is preceded in English indefinite constructions by modifiers such as some or much rather than a or one.

Wiktionary

1. n. A noun that normally cannot be counted.

WordNet 3.0

1. n. a noun that does not form plurals