It stands for midpoint liquidity; a market liquidity that is provided by trades/ orders executed, or to be executed, at the midpoint. Midpoint liquidity is considered the most stable liquidity market-wide, as traders are less likely to experience adverse selection (for prices at which buy or sell orders would be filled).

Orders are pegged to the average of the National Best Bid (NBB) and National Best Offer (NBO), collectively, NBBO. The price in a midpoint pegged order is determined as the less aggressive of one minimum price variant (MPV), i.e., $0.01 for most stocks, lower than the NBB for buy orders (or higher than the NBO for sell orders) or the order’s limit price. The order combines specific features of multiple orders including primary peg order and discretionary order types.

Midpoint liquidity is made and enhanced by trades being executed, or available for execution, at the midpoint of the protected best bid or offer (PBBO)- where for a midpoint liquidity order (MPL order) (being a limit order to buy (sell) a security (or a tradable asset) that is not displayed and automatically executes at the midpoint), a working price is filled at the lower (higher) of the midpoint of the protected best bid and offer (PBBO) or its limit price.