The mid price is the average of the best bid price and the best ask price quoted for a security at a given moment:
With a bid of 100.10 and an ask of 100.14, the mid price is 100.12. It is treated as the best simple estimate of what the security is worth at that instant, because it removes the cost of crossing the bid-ask spread. Nobody can actually trade at it in a normal order book: a buyer pays the ask and a seller receives the bid. The mid price is the usual reference for valuing positions, for quoting options, and for transaction cost analysis, where the gap between the execution price and the mid at the time of the order measures cost and slippage. A refinement weights the two prices by the bid size and ask size, so that the estimate leans towards the side with less quantity resting on it.