A definition is an explanation of what a word, phrase, or concept means. When you look up photosynthesis in a science textbook, the definition tells you it's the process plants use to turn sunlight into food. When someone asks, “What does perseverance mean?” they're asking for a definition.

A good definition provides full understanding: it helps you grasp what something is, how it works, when to use it, and sometimes what it isn't. If you defined brave as simply “not afraid,” you'd miss something important: bravery means acting courageously despite being afraid, not never feeling fear at all.

Definitions appear in dictionaries, textbooks, and everyday conversation. Scientists write careful definitions to make sure everyone understands technical terms the same way. Teachers often start lessons by defining key concepts. Even friends might pause to define what they mean by words like fair or fun when they're making plans together.

A definition sets boundaries around a word's meaning, showing where it applies and where it doesn't. Without definitions, communication would be difficult. People would use words differently, sometimes talking past each other without realizing it.