The Invisible jet was an airplane used by Wonder Woman.
After the Thanagarian invasion, Wonder Woman acquired a cloaked jet for her private use. Both its origin and the technology behind it remain a mystery.
Apart from flight, it also had the capability to travel underwater.
Pre-Crisis Wonder Woman didn't possess the ability to fly, so the Amazons made her a plane. Originally a propeller aircraft, it changed into a jet as real-world technology advanced. Post-Crisis, it was actually a gift by the alien Lansinarians, and had a symbiotic link with Wonder Woman.
The DCAU's plane's origin was supposed to have been related in Justice League: Worlds Collide. Though that project was revised into the script for Justice League: Crisis On Two Earths, the latter movie gives some indications on what would have been the plane's origin. On a parallel Earth, Wonder Woman stole Owlman's plane, which had a cloaking ability. A fight with Superwoman left the plane jammed in its cloaked mode. After the adventure, she took it home.