This is the principle, first stated by Heisenberg, that at the atomic level and deeper in the structure of matter, it is not possible to know exactly the states of complementary parameters, such as the position of a particle and its momentum, at the same time. More specifically, as a consequence of the uncertainty principle, if the state of one parameter is known, the complimentary parameter is in an infinity of states. Therefore, it is impossible to fully predict the behaviour of a subatomic system. As one result of the uncertainty principle, the principle of causality that seems to apply at macroscopic level cannot, even in principle, apply at the atomic level and deeper. To find out more about Heisenberg and the uncertainty principle go to links. 
