Uncountable describes things you usually don't count as separate items because they don't come in individual pieces. Water is uncountable: you don't normally say “one water, two waters, three waters” the way you can say one apple, two apples, three apples. Instead, you measure water by volume (cups, gallons, liters) or talk about it as a whole: “some water” or “a lot of water.”

Many substances work this way: sand, sugar, air, milk, flour. You might count containers of these things (three bags of flour, two glasses of milk), but the substances themselves blend together as continuous material.

In grammar, uncountable nouns (also called mass nouns) behave differently from countable ones. You say “much water” not “many waters,” and “less sugar” not “fewer sugars.” This matters when you're writing or speaking precisely.

Some words can be both countable and uncountable depending on meaning. “Paper” is uncountable when you mean the material (recycling paper saves trees), but countable when you mean individual documents (turn in your papers tomorrow).

The opposite of uncountable is countable (also called count nouns): distinct items you can number, like books, chairs, or ideas.