Quick definition
The gap between the highest price a buyer is willing to pay (bid) and the lowest price a seller will accept (ask). In options, the bid-ask spread is a direct tax on entering and exiting positions.
The round-trip cost
A trader who buys at the ask and sells at the bid pays the full spread as a cost of doing business. On a $0.05-wide spread that is 2.5 cents each way; on a $0.50-wide spread, this becomes real money and can dwarf the theoretical edge on many setups. Options traders who screen only on Greeks and ignore spreads reliably underperform their own backtests.
Mid-price fills and reality
Most brokers let traders submit limit orders at the mid-price. On liquid names those often fill; on illiquid names they sit forever, and the trader eventually walks the order toward the ask to get filled. Real execution price is somewhere between mid and ask on entry, mid and bid on exit — factoring this in is part of honest backtesting.
How Treeova uses it
Treeova's execution layer prices setups using slippage-adjusted quotes rather than raw mid-price, so an agent's projected fill matches what actually happens at the broker. Arch-AGI also filters candidate strikes on maximum acceptable spread — a theoretically clean iron condor on wide-spread legs is not a clean trade in practice.