The price at which an order fills immediately — the ask if you are buying, the bid if you are selling, or the sum of those on a spread.
The natural is the guaranteed-fill price. It is what you pay for certainty, and on a multi-leg order it can be dramatically worse than the mid-price because you cross every leg's spread at once.
Traders quote both figures before entering: "natural 2.40, mid 2.10." The gap is the negotiating range, and on illiquid spreads it is often larger than the edge in the trade itself.
Example: an XYZ vertical-spread where the long leg quotes 3.00 / 3.30 and the short leg quotes 1.40 / 1.60. Natural debit is 3.30 − 1.40 = 1.90. Mid debit is 3.15 − 1.50 = 1.65. Paying natural costs $25 per spread more, or $250 on a ten-lot.
Original diagrams for the ideas on this page. Illustrative, not real market data.
The bid-ask spread. Buy orders sit below, sell orders above, and the gap between the best bid (50.01) and best ask (50.04) is the spread you pay to cross. Bar length shows the size resting at each price.
Educational only, not advice. Spotted an error? Post in Site Feedback.