The term SEAMAN'S 'STOP!' is a command used by a seaman, which is someone who works on a ship. The crossword solutions for this term are AVAST and BELAY, both of which are five letters long. AVAST is an order to stop or cease what one is doing, while BELAY is an order to secure a rope or line. Together, these two commands form the phrase SEAMAN'S 'STOP!' which is used to indicate that all activity on the ship should cease immediately.