Extensively means thoroughly, in great detail, or covering a wide area. When you study a topic extensively, you don't just skim the surface: you dig deep, exploring many different aspects and sources. A scientist might research extensively before conducting an experiment, reading dozens of articles and running numerous tests to understand every angle of the problem.

The word suggests both depth and breadth. If archaeologists excavate extensively at an ancient site, they're carefully examining a large area over a long period. If a doctor questions you extensively about your symptoms, they ask many detailed questions to understand exactly what's wrong.

You might read extensively about Ancient Egypt, devouring books about pharaohs, pyramids, daily life, and hieroglyphics. Your friend might only read one book on the topic. The difference is in how much ground you covered and how deeply you explored.