What is "clipping"?

I've seen several references to "clipping" involving amplifiers.

What does clipping mean?

The term "clipping" refers to the amplifier's electrical condition when driven beyond it's maximum "clean" output level. All amplifiers have positive and negative "rails" in the power supply, and depending on signal conditions and levels, some or all of this rail voltage is used. When asked to deliver more power than the amplifier is normally capable of producing, it will "clip off" the top and bottom of the signal. This appears as a waveform that is somewhat flat at the top and the bottom of the sine wave when viewed on an oscilloscope. The clipped output has two distinct undesirable properties: distortion and excessive power.

While the distortion produced by clipping doesn't cause any damage to either speakers or amplifiers (it is just "dirty" or "sloppy" sounding), the excessive power produced when driving the amplifier to that level can cause speakers to fail quite easily! A heavily-clipped amplifier will develop up to 2 times it's rated power, and will draw excessive amounts of current through the amp's power supply as a result. Over time, this will begin to damage both the amplifier and the speakers that are connected to it. Some keys to recognizing an amplifier that is clipping is to remember that if the amplifier shuts down regularly and runs very hot to the touch, it is probably not set up correctly and is heavily clipping as a result. 

There is misinformation circulating that some say clipping is actually "DC" (Direct Current) because it "resembles" the flattened parts of the sinewave represent positive and negative conditions.  This is not true and is not the same as a a sinusoidal image (sine wave) under clipped conditions.