rpenmanparker is correct in his definitions of accuracy and precision in the statistical and scientific fields. Precision relates to spread, and accuracy relates to the position of group center. Lack of accuracy, or bias, is easy to fix. You just adjust your sights. Lack of precision is usually harder to fix. That relates to bedding, barrel quality, etc.
It's important, because if precision is lacking, it is difficult to say with certainty where the long-term group center is. That limits your ability to zero your sights. Get precision first, then worry about accuracy.
The other one term that is often used incorrectly is velocity. When you use a chronograph, you are measuring speed, which is a scalar quantity that requires only one number to represent. Velocity is speed plus direction, and requires three numbers to represent, one each for the X, Y, and Z axes.