To give a layman's definition:

"You cannot measure/observe something without changing that which you are measuring/observing."

Romantics and artists find this appealing because it appears to refute scientific method. This is far from the truth. It simply means you must account for it.

See also:


Source: Hawking, Stephen, "A Brief History of Time", Bantam Books, NY, 1988 Last Updated 12.30.03