What is Buy Minus Order?

In the context of general equities, rare market or limit order to buy a stated amount of a stock, provided that the price to be obtained is not higher than the last sale if the last sale is a minus or zero-minus tick, and is not higher than the last sale minus the minimum fractional change in the stock if the last sale is a plus or zero-plus tick. (If limit, then the buy cannot occur above the limit, regardless of tick.)