It stands for market maker peg order; a limit order that, once placed/ entered, the bid or offer is automatically priced by the system, as per the applicable quotation requirements for market makers (MMs), at the designated percentage away from the then current national best bid and national best offer (NBBO), or if no national best bid (NBB) or national best offer (NBO), at the designated percentage away from the last reported sale transaction from the responsible single plan processor for the respective day, or if none, the previous closing price as adjusted to account for any corporate actions (such as dividends or stock splits) involving the traded security.

This order allows a market maker to maintain a continuous two-sided quotation at a displayed price as per the quotation requirements. The displayed price of this order is pegged to to a certain reference price in order to keep the displayed price within a bounded price range. The order must be placed with a limit price beyond which it may not be priced.