When something is far-reaching or expansive it is extensive. When your friend knows every single place and time that his favorite band has played any given song, he has extensive knowledge of that band.

Use the adjective extensive for something that goes on and on and on. It works for anything that covers a lot of territory either in physical space, as in "the gardens at the palace are extensive," or conceptually, as in "her knowledge of the mortgage-backed securities market is extensive." If you are a farmer, you would also use extensive to mean that you are able to work a lot of land without too much effort or manual labor.

Definitions of extensive

adj large in spatial extent or range or scope or quantity

“an extensive Roman settlement in northwest England”
“they suffered extensive damage”
big, large
above average in size or number or quantity or magnitude or extent

adj broad in scope or content

adj of agriculture; increasing productivity by using large areas with minimal outlay and labor

“producing wheat under extensive conditions”
“agriculture of the extensive type”
of agriculture; intended to increase productivity of a fixed area by expending more capital and labor

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