Countable describes things you can count one by one: 1, 2, 3, 4, and so on. Apples are countable because you can say “one apple, two apples, three apples.” Books, chairs, people, and days are all countable.

Some things aren't countable because they don't come in separate units. You can't count water the same way: you don't say “one water, two waters.” Instead, you measure it: “two cups of water” or “a gallon of water.” The same goes for sand, air, or happiness. These are uncountable or mass nouns.

The distinction matters in grammar. With countable nouns, you say “many books” or “fewer chairs.” With uncountable nouns, you say “much water” or “less sand.” When you're writing or speaking carefully, this difference helps you choose the right words.

In mathematics, countable takes on a more technical meaning. Mathematicians say a set is countable if you could theoretically match each item with a counting number, even if the set goes on forever. The set of all whole numbers (1, 2, 3...) is countable because you can list them in order. This concept becomes important in advanced mathematics, where some infinities are actually larger than others.