The mid price is the average of the best buy price (bid) and best sell price (ask) on an issue.
In plain terms — Buyers offer €99.90, sellers ask €100.10. The mid is €100.00 — the «middle». It is often the indicative price you see, but not what you pay if you click buy at market.
$$\text{mid} = \frac{\text{bid} + \text{ask}}{2}$$
Mid vs what you actually pay
| Action | Typical fill price |
|---|---|
| Buy at market (market-order) | You pay the ask (sellers' side) |
| Sell at market | You receive the bid (buyers' side) |
| Chart / ticker | Often mid or last trade |
That is why slippage includes the bid-ask-spread: expecting mid but filling at ask is already a gap.