A locked market refers to a situation where the bid and ask price for a security is identical. This is an abnormal market condition—the bid price will always be below the ask price in normal trading conditions. Locked markets occur due to the complexity of modern financial markets.
A crossed market is the name traders and market makers give to a circumstance where a market's bid price exceeds its ask price. This is an unusual circumstance made even more rare with the advance of electronic and computerized trading.