It's the difference between the bid and ask price offered. Liquid contracts or contracts with a higher trading volume mostly have a tighter Bid/Ask spread, and a contract that has a lower trading volume generally have more room between the Bid and Ask price.[click to read more]
The characteristic of high volume of trades in an option so as to be able to easily square of any held position in that option at reasonable market price. Volume and the width of the bid-ask spread are good idications of overall contract liquidity. [click to read more]
The Call Volume All Strikes is the sum of all call options volume across all available strikes and expiration dates. Volume is the number of contracts for a specific strike and expiration that traded hands.