Thirty is the number that comes after twenty-nine and before thirty-one. It's written as 30 in numerals and represents three groups of ten, or three decades.

In everyday life, thirty shows up constantly. A month usually has about thirty days (some have 31, February has 28 or 29, but we often round to 30). Many classrooms have around thirty students. If you're counting by tens, you go 10, 20, 30, 40. When you multiply 3 × 10 or 5 × 6, you get thirty.

The word can describe age too. When adults turn thirty, they've lived three full decades. Some people make a big deal about turning thirty because it feels like leaving youth behind, though thirty-year-olds would probably tell you they still feel pretty young.

In sports, particularly tennis, thirty is a score: if you win two points in a game, the umpire calls out “thirty.” And you might hear the phrase thirty-thirty, which describes a tied score where both players have won two points each.