I got curious about nihilism and went to find out more about it. The closest I can come to an "official" definition that's also credible (sorry, Wikipedia) is this:
Nihilism is the belief that all values are baseless and that nothing can be known or communicated. It is often associated with extreme pessimism and a radical skepticism that condemns existence. A true nihilist would believe in nothing, have no loyalties, and no purpose other than, perhaps, an impulse to destroy.
I found that to be a little extreme, since any philosophy that truly negates knowing anything has already negated itself, if it claims to know anything.
Then there was this, which seemed semi-apocalyptic:
Values and purpose are human inventions designed to be shared between us. Like language, values and purpose only work if we all know and agree on what they mean. They are easily manipulated by changing meaning without changing the symbol for it.
There was a more sensible definition elsewhere:
Nihilism is the perception that no inherent value exists.
Nihilism at Nihil.org: Center for the Nihilism and Nihilist Studies
Basically, it's a belief that there is no singular Cause or Truth in the universe, just our interpretations -- some of which are right, some of which are crap, in varying degrees.