This is something that is hard to explain, and easy to understand when explained RIGHT. I'm searching on the internet for a decent explanation because my words are junk, but here is one definition...

The attenuation that occurs when two waveforms of equal frequency and opposite polarity are combined. The attenuation may be total when the waveforms are also of equal amplitude.

http://www.recordingeq.com/articles/321eq.html

or

http://www.sfu.ca/sca/Manuals/ZAAPf/p/phase.html

These sites should help you out with definition and understanding. After you've got a grasp of it there are tons of posts here in recording.org on controlling/eliminating phase cancellation.

I hope I helped.

Mostly people mean polarity when they use the word phase.

Phase is frequency conscious.

Hey Phantom,

In a real world sense . . . here's my explanation.

If you think of the effect of a guitar Phase Shifter stomp box . . .
when I have phasing issues on drum overhead mics for example,
it literally sounds like they're being subtly run through a Phase
Shifter. OR . . . it can also cause the volume (attenuation) of the
combined two mics to drop. This is because when there are phasing
issues, certain frequencys are cancelling each other out.
Typically the phasing one encounters due to poor mic placement
is not desirable. Also, some EQ settings can add or remove phasing.
It's often recommended that you spend a little time listening back
to you mix in mono and see if the volume of certain intruments
seem louder or are disappearing in the mix. This can indicate phasing
issues which can usually easily be adjusted through EQ.

Ironically, some kinds of phasing can be desirable . . . for example
DUY makes a Pro Tools plugin I have called WIDE. When placed on
Master Fader in Pro Tools it can add phasing to the mix which can
give the mix the feeling that the stereo separation is wider than
before.