What is Vulcan?

Vulcan definition and meaning on Dictionary terms:

the ancient Roman god of fire and metalworking, identified with the Greek Hephaestus.
Military. a six-barrel, 20mm U.S. Army antiaircraft gun system mounted on an armored personnel carrier and first deployed in 1968.
Astronomy. a hypothetical planet nearest the sun whose existence was erroneously postulated to account for perturbations in Mercury’s orbit.