Extensive means covering a large area, amount, or range. When something is extensive, there's a lot of it or it reaches far and wide.

A library might have an extensive collection of books on dinosaurs, meaning shelves and shelves full of every kind of dinosaur book you could imagine. A detective conducting an extensive search checks every drawer, looks under furniture, and examines every corner carefully. When a scientist does extensive research, she spends months or years studying her subject from every angle.

The word suggests thoroughness and scope. An extensive repair to a house involves replacing the roof, updating the plumbing, and repainting multiple rooms, not just fixing one broken window. A coach's extensive experience means decades of working with hundreds of players in countless situations.

You might hear people describe extensive damage after a storm, meaning the destruction spread across a wide area. Or someone might have extensive knowledge of ancient Egypt, knowing not just about pyramids but also about daily life, farming methods, religious beliefs, and warfare.