Open Interest
The total value of derivative contracts on an asset that are currently open — neither closed out nor settled. It counts positions, not trades.
Volume says how much changed hands; open interest says how much is still outstanding. Rising price with rising open interest means new money is taking positions, while rising price with falling open interest means existing positions are being closed — the same price move with two different populations behind it. Available only for assets with a derivatives market, which is why the fundamentals panel shows it for some and not others.