Waaagh! is the psychic field that Orks in Warhammer 40k produce. The psychic energy produced in the Waaagh! is the main reason that Ork technology even functions, as without it, most of their weapons and vehicles would be unable to work. The Orks also use it as a battlecry.
The word is always spelled with at least three As, though more are used when expressing an especially long or excited battlecry.
'Ey Boyz, we gotz da Waaagh! on our side, we ain't losin' ta no humies.