Uncertainty principle (W. Heisenberg; 1927): A principle, central to the quantum mechanics that states which two complementary parameters (like energy and time, position and momentum, or angular momentum and angular displacement) can’t both be recognized to the infinite accuracy; the more you know regarding one, the less you know regarding the other.
It can be exemplified in a fairly clear manner as it relates to position versus momentum: To see something (let's state an electron), we have to fire the photons at it; they bounce off and come back to us, therefore we can "see" it. When you select low-frequency photons, with a low energy, they do not impart a lot momentum to the electron; however they give you a very fuzzy picture, therefore you have a higher uncertainty in position and hence you can contain a higher certainty in the momentum. On other hand, when you were to fire very high-energy photons (that is x-rays or gammas) at the electron, they would provide you a very apparent picture of where the electron is (that is, higher certainty in position), however would impart a big deal of momentum to the electron (that is, higher uncertainty in the momentum).
In a more generalized intellect, the uncertainty principle states us that the performance of observing modifications the observed in primary way.