An uncountable noun, also known as a mass noun or non-count noun, refers to a noun that represents something that cannot be easily counted or quantified as discrete units. These nouns typically describe substances, concepts, or abstract ideas that are viewed as indivisible or continuous. Uncountable nouns cannot be used in the plural form or be preceded by "a" or "an" because they lack a specific or countable quantity.

Examples of uncountable nouns include:

1. Water: You cannot say "a water" or "two waters." It is an uncountable noun because it represents a substance that cannot be counted in individual units.

2. Knowledge: While you can possess knowledge, you cannot count it. You cannot say "one knowledge" or "five knowledges."

3. Advice: You can give or receive advice, but you cannot count it. You wouldn't say "an advice" or "three advices."

4. Happiness: Happiness is an abstract concept that cannot be measured in discrete units. You cannot say "a happiness" or "many happinesses."

5. Information: Information is another example of an uncountable noun. You can have a lot of information, but you can't count it as individual pieces.

6. Furniture: This noun represents a category of objects that cannot be counted individually. You wouldn't say "a furniture" or "three furnitures."

To refer to an indefinite or specific amount of uncountable nouns, you can use quantifiers such as "some," "a lot of," "a little," "much," or expressions like "a glass of water," "a piece of furniture," or "a bit of knowledge."