JET, Jet or Jets all have many meanings. Some of them are:

Science
A jet is a stream of liquid or gas which is moving together in parallel.
In fluid or gas engineering contexts, a jet is a stream of material ejected from a nozzle, or a nozzle designed to produce such a stream.
In automotive terms, a jet is a tube, nozzle, or passageway through which petrol is metered within a carburetor.
In geology, jet (lignite) (or Whitby jet) is a mineraloid (mineral-like material) which is used as a gemstone.
In meteorology, a blue jet is a form of upper atmospheric lightning.
In meteorology, a jet is an electrical discharge that occurs high above the cumulonimbus cloud of an active thunderstorm.
In astrophysics, a relativistic jet is a stream of fast-moving plasma emitted at nearly 300,000 km/s (the speed of light) from an active galaxy. A similar phenomenon on a much smaller scale is a polar jet.  These are emitted during star formation from either side of the accretion disk, along the two polar axes.
In nuclear and particle physics, a jet is a cone of hadrons
In nuclear physics, the Joint European Torus or JET is an experimental Nuclear fusion reactor.
In mathematics, a jet is an operation that produces an abstract polynomial
In mathematics, a jet bundle (or spray) is a manipulation of differential equations
In software engineering, Java Emitter Templates (JET) is a Template engine that is part of the Eclipse EMF project.

Aerospace
Jet engine
Jet aircraft
Jet Airways, an airline based in India serving domestic and international routes
JetBlue Airways, an airline based on New York
Jetstar Airlines, an Australian airline servicing domestic and International routes

Water
Water jet

Music
Jet, a hit rock music single on the 1973 album Band on the Run by Wings
Jet-CD (also known as Jet), 1998's album by J-pop band, Puffy AmiYumi
Jet, a rock band from Melbourne, Australia
Jet, a rock band that existed from 1971 to 1974, from Genoa, northern Italy
Jet, a glam rock band from England that recorded one eponymous album in 1975
The Jets, a family band from Minneapolis, Minnesota that does pop, R&B, dance and religious music, and had several hit singles in the 1980s
The Jets, a 1970s rock band from Pekin, Illinois
Jet Records, a record label

Fiction
Jet, a character from Wildstorm/DC Comics, and the daughter of Backlash. Another character by the name of Jet, a female one, exists in DC comics, New Guardians.
Jet Black, a main character from the anime Cowboy Bebop.
Jet, a character from the Earth Kingdom forests in the TV series Avatar: The Last Airbender
The Jets, a gang in West Side Story which fought with the Sharks.
JET TV, a television station in the Jadeworld station group
Jet the Hawk, a video game character.
Jet, an addictive drug in the Fallout series of video games.
Jet Fusion, a super spy who makes occasional appearances in the cartoon The Adventures of Jimmy Neutron: Boy Genius

Sports
New York Jets, a professional American football team in the National Football League
Newcastle United Jets, a professional soccer team in the Australian A-League competition
Winnipeg Jets (1972–96), a former professional hockey team in the World Hockey Association and National Hockey League
Winnipeg Jets, a professional ice hockey team in the National Hockey League
Jet (Diane Youdale) was one of the stars of Gladiators
"Jet," or "Jet Terry," nickname of the popular Dallas Maverick and former University of Arizona point guard Jason Terry.
Benny "The Jet" Urquidez, A kickboxer, coreographer and actor (Benny Urquidez)

Culture
Jet (slang), to leave in a hurry
Jet, an African-American themed magazine
Jet, a filling station brand of ConocoPhillips
A ZX Spectrum clone from Romania.
 JET, a 1985 flight simulator produced by subLOGIC
 The JET Programme - The Japan Exchange and Teaching Programme (teaching English)

Places
Jet, a town in Alfalfa County, Oklahoma, United States