A market in a particular stock or option is described as crossed when a bid to buy that stock or option is higher than the offer to sell it, or when an offer to sell is lower than a bid to buy.
A crossed market reverses the normal relationship of a stock quotation in which the bid price is always lower than the ask price. It's illegal for market makers to cross a market deliberately.
A crossed market may occur when investors place after-hours orders electronically for execution at opening, or when investors trade directly through an electronic communications network (ECN).
NASD has introduced a set of pre-opening procedures for market makers on the Nasdaq Stock Market. They help prevent the confusion and potential inequalities in pricing that a crossed market can produce.