A concept is a mental grouping of similar objects, events, ideas, or people. It is the basic unit of thought that allows us to treat different instances as members of the same category.

Some concepts are defined by clear rules—triangles have three sides—but most everyday concepts are organized around prototypes or typical examples. A robin is a more prototypical bird than a penguin; people classify the robin faster and with more confidence. Concepts can also be represented by collections of exemplars actually encountered, or by theories about the underlying properties that hold a category together. Hierarchical organization is common: a collie is a dog, a dog is an animal, an animal is a living thing.

Concepts reduce the complexity of the world. Once an object is categorized, knowledge already stored about the category becomes available—what it is for, how it behaves, whether it is safe. Language both reflects and shapes conceptual structure; different languages carve the world into somewhat different categories. Concept learning continues throughout life as new experiences are assimilated or force the revision of existing categories.

Without concepts, every encounter would be unique and nothing could be recognized or predicted.