yellow flag

noun

• 1a ship’s yellow flag, denoting the letter Q for ‘quarantine.’ When flown with another flag, it indicates disease on board; when flown alone, it indicates the absence of disease and signifies a request for customs clearance. Also called quarantine flag.
• Auto Racing a yellow flag used to signal to drivers that there is a hazard such as oil or a crashed car on the track.

• 2a yellow-flowered iris that grows by water and in marshy places, native to Europe and naturalized in North America.

  • Iris pseudacorus, family Iridaceae