blue light

Short definition from Wiktionary

Noun

1. 1
   A mixture of chemicals (including nitre, sulfur and antimony) used in the eighteenth and nineteenth centuries for night-time signaling and general illumination.
2. 2
   A New England federalist, who opposed the Anglo-American War of 1812; (loosely), a federalist.
3. 3
   A flashing light, usually fitted to an emergency vehicle.
4. 4
   Used other than figuratively or idiomatically: see blue, light.

Verb

1. 1
   To travel quickly in a police or ambulance vehicle with the lightbar (and possibly the siren) activated.