Mid-Price
The average of the best bid and best ask, used as a reference price for quoting and PnL calculation.
Definition
The mid-price is calculated as (best bid + best ask) / 2. It represents a theoretical fair value between the two sides of the market. Makers typically quote symmetrically around the mid-price, and bots use it as a reference for PnL calculations and for detecting whether a market has moved in their favor.
In practice
If the Polymarket YES order book shows a best bid of $0.48 and a best ask of $0.52, the mid-price is $0.50. A maker bot posting a bid at $0.488 and an ask at $0.512 is quoting 1.2 cents inside the mid on each side. When Binance price action shifts, the bot updates its mid-price estimate and repositions its GTC orders. Unrealized PnL for a filled position is usually calculated relative to the current mid-price rather than the filled price.