Noun

1. A blue signal flag with a white rectangle in the centre, signifying "P". When flown alone, indicates that a ship is ready to sail, requiring all crew members and passengers to return on board.
   • We lay out in the stream for another whole day, with the Blue Peter flying, to show that we were ready for sea, and to summon any passengers who might yet remain on shore. — W.H.G. Kingston, "Peter the Whaler", 1851