A non-countable noun, or mass noun, is a noun which does not have a plural form. (Non-countable nouns refer to things that cannot be counted.)
music, furniture, grit, mercury
Non-countable nouns are always singular.
Amount, quantity and number What are nouns?
The different types of nouns
What are pronouns?
The different types of pronouns
Verbal nouns (gerunds)
Click on the non-countable noun: