noun

• (astronomy) A small Solar System body consisting mainly of volatile ice, dust and particles of rock whose very eccentric solar orbit periodically brings it close enough to the Sun that the ice vaporises to form an atmosphere, or coma, which may be blown by the solar wind to produce a visible tail.
• A celestial phenomenon with the appearance of such a body.
• Any of several species of hummingbird found in the Andes.
• The fifth reindeer of Santa Claus.
• (programming) A web application model in which a long-held HTTP request allows a web server to push data to a browser, without the browser explicitly requesting it.