It stands for contra midpoint only; a non-displayed order type that is priced at the midpoint between the best bid and the best offer (e.g., NBBO). A contra midpoint only (CMO) resting on the order book may be subject to cancellation by an exchange upon placement of certain types of orders that may indicate less favorable pending price movements than the prevailing price, thus providing protection to the CMO trader against fillings at the prevailing less favorable midpoint price.
Upon cancellation, the trader would be free to submit a new CMO at the new (more favorable) midpoint price or, in certain situations, the exchange would automatically place new orders on behalf of the trader.
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