It is impossible to define both the parameter and its canonical conjugate to arbitrary accuracy. This means that two coordinates which are canonical conjugate of each other (whose canonical commutation is non-zero, or whose Poisson bracket is non-zero) can not be simultaneously measured with arbitrary accuracy.

Examples:

1. \({x,p}!=0\), or \([x,p]=i\bar{h}\). So we can not define position and momenta to arbitrary accuracy.