A category is a group of things that share something important in common. When you organize your books by category, you might put all the mysteries together, all the science books together, and all the graphic novels together. Each category contains items that belong together because of shared characteristics.

Think about a library: fiction and nonfiction are broad categories, but within fiction you'll find smaller categories like fantasy, historical fiction, and realistic fiction. Scientists use categories constantly, organizing animals into categories like mammals, reptiles, and birds based on their traits. Your brain uses categories automatically: when you see a new four-legged furry animal, you quickly categorize it as either a dog or a cat based on what you already know about those categories.

Categories help us make sense of a complicated world. Instead of treating every single thing as completely unique, we notice patterns and create categories that help us think, communicate, and make decisions more efficiently. When a teacher asks you to “categorize these words by parts of speech,” you're sorting them into groups like nouns, verbs, and adjectives.

The verb form is categorize, which means to sort things into categories. Something described as categorical means absolute and without exception, like a categorical refusal that leaves no room for debate.