Mathematically expressed as:

   dx dp >= -

(Okay, yeah, this is supposed to say "delta-x times delta-p is greater than or equal to h-bar over 2", but trying to express mathematical formulae in HTML sucks eggs. I can't wait until Mozilla or something learns how to grok MathML. Or TeX.)

h-bar is Planck's constant

Essentially what this means is that the more precisely you know something's position (x), the less you know about that object's momentum (p) and vice versa.

There's a nice web page about the Heisenberg Uncertainty Principle at