blue light

noun

• A pyrotechnic preparation which burns with a blue flame and is used to send signals, especially at night-time.
• (chiefly derogatory). A New England federalist who opposed the Anglo-American war of 1812–15; (later more generally) a federalist, a person perceived as disloyal. Frequently as a modifier.
• A blue flashing light used on a police car, ambulance, or other emergency vehicle, typically when responding urgently to an incident.