A specific brand of Japanese toy laser gun, marketed in the mid-1980's. The game of which these games were the integral element became such a huge sensation that it spawned a series of Metroid-ish console games (programmed by Sega) and a sequence of anime films (those based more specifically off of Sega's game series). The Zillion pistol was the model for the Master System's light gun, imaginatively named the Light Phaser.
[Ed. Note 4/6/2002 (Gz): removed reference to presumed deleted writeup]