To classify means to sort things into groups based on what they have in common. When a librarian classifies books, she organizes them by subject: all the mystery novels together, all the science books together, all the history books together. When a scientist classifies animals, he groups them by shared traits: mammals in one category, reptiles in another, insects in a third.

Think of your teacher asking you to classify a pile of shapes. You might put all the triangles in one group, all the circles in another, and all the squares in a third. You're creating categories based on each shape's characteristics.

The noun form is classification. The animal classification system groups living things from broad categories (like “animals”) down to specific ones (like “golden retriever”). Scientists use classification to make sense of millions of species. Libraries use classification systems like the Dewey Decimal System to help people find books.

When something is classified, it has been sorted into a category. In government and military contexts, classified has a special meaning: information labeled as secret and restricted to people with proper clearance. A classified document contains information the government doesn't want the public to see. When information is declassified, it is no longer secret.