Open Interest
The total size of derivative contracts currently open, counting positions that have not yet been closed or settled.
Open interest rises when a new position is created between two parties and falls when a position is closed, expires, or is liquidated. It is reported per venue and per contract, sometimes denominated in the underlying coin and sometimes in dollars, so figures are not directly comparable across venues without care. Read alongside funding rates, it describes how much leveraged exposure is outstanding and on which side it is crowded. A sharp fall in open interest during a price move usually indicates positions being closed or liquidated rather than new activity.
In practice
One contract traded back and forth between the same two traders many times generates volume repeatedly while leaving open interest unchanged.
The common misunderstanding
Open interest is not trading volume: volume counts every trade, while open interest counts only positions that remain open.