A subset is a group that fits entirely within a larger group. Every member of the subset also belongs to the bigger set, but the bigger set usually contains additional members too.

In mathematics, if you have a set of all the students in your school, the fifth graders form a subset because every fifth grader is also a student in the school. The numbers 2, 4, and 6 form a subset of all even numbers. Your collection of baseball cards is a subset of all baseball cards ever printed.

The word appears in everyday situations beyond math class. Dog breeds are subsets of all dogs. Mystery novels form a subset of all novels. The students who play in the school band are a subset of all students.

Sometimes people talk about proper subsets versus improper subsets. A proper subset must be smaller than the original set (fifth graders versus all students). An improper subset can actually equal the whole set, which sounds strange but matters in formal mathematics.

Understanding subsets helps you organize information and see how categories relate to each other. When someone says “reptiles are a subset of animals,” they're pointing out that while all reptiles count as animals, plenty of animals (like birds and mammals) aren't reptiles. The subset relationship flows one direction: the smaller group fits inside the larger group, not the other way around.