Liquidity
The ease with which an option can be bought or sold without significantly affecting its price.
Explanation
Highly liquid options have tight bid-ask spreads, high open interest, and high volume. Liquidity is crucial for getting favorable fills and being able to exit positions efficiently. SPY, QQQ, AAPL, and other large-cap names tend to have the most liquid options. Low liquidity leads to slippage and wider spreads.
Example
SPY options often have a bid-ask spread of $0.01-$0.02, while a small-cap stock might have spreads of $0.50 or more.