Bid-ask spread
The gap between the bid and the ask. Wider spreads raise the cost of entering and exiting a contract, and thinly traded options tend to have wider ones.
The spread is the difference between the price buyers are posting and the price sellers are posting. A trader who buys at the ask and later sells at the bid gives up the full spread on the round trip, before commissions and fees.
Spreads tend to be narrow in actively traded contracts on large underlyings and wide in far-dated, far out-of-the-money or thinly traded contracts. Measuring the spread as a percentage of the option price makes contracts with different prices comparable.
Illustrative example (hypothetical numbers, not a real trade): an option is quoted $0.90 bid and $1.10 ask. The spread is $0.20, which is 20% of the $1.00 midpoint. On one contract, buying at the ask and selling at the bid with the underlying unchanged would cost 0.20 × 100 = $20.