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1.
Flashlight (noun)
A Photon is a subatomic light wave/particle.
A Cannon is a device that shoots something.
Thus a Photon Cannon is a device that shoots Light.
Protoss Player <to all>: "HAHAHAHA! my dragoons just pwned ur hydras like there all photophobic! ROFLMA"--NUCLEAR LAUNCH DETECTED.--"OH SHIT!"
<BOOM>
Zerg Player <to Terran ally>: "gj"
Terran Player <to Zerg ally>: "thx hehe >:)"
by etox May 15, 2004
 
2.
A device using photon particles being repelled via gravitational array reflection to supercharge and heat a space.
A device that can be attached to a craft to superheat the atmosphere and or space area to create an artificial rip in the space time continuum.
by Jonathan Walston May 30, 2005
 
3.
The big ass gun that the guy from Marvel vs. Capcom 2 uses.
"Yo you know what they call that big ass gun... a Photon Cannon"
by Alp December 11, 2004