The more precisely the position is determined, the less precisely
the momentum is known in this instant, and vice versa.
~Werner Heisenberg

This is a succinct statement of the "uncertainty relation" between the position and the momentum (mass times velocity) of a subatomic particle, such as an electron. This relation has profound implications for such fundamental notions as causality and the determination of the future behavior of an atomic particle.

Because of the scientific and philosophical implications of the seemingly harmless sounding uncertainty relations, physicists speak of an uncertainty principle, which is often called more descriptively the "principle of indeterminacy."
*This page focuses on the origins of Heisenberg's uncertainty relations and principle.