The mid-price of a security is the average of the the bid and offer prices. This may be as offered by market makers or in the order book.
The mid-price is frequently quoted when it is necessary to provide a single number for a security price. It has the advantage over the last traded price of a constantly updated quote being available for securities that are traded through market makers.
The main disadvantage of quoting the mid-price is that,for very illiquid securities trading through matched bargain systems, the bid or offer price may be unrealistic and distort the mid-price. For these securities, it is preferable to use the last traded price.