Precision and Accuracy
Precision and accuracy are often used synonymously but they are in fact different things. It is possible to have very precise measurements that are not at all accurate. Here's a good example to share with your geoscience students.
Imagine you want to measure the arrival time of a seismic wave on a seismogram. You may carefully measure the time on the seismogram several times and produce multiple measurements that are all very close to one another. This set of measurements would be very precise. Now imagine that the clock you used to record the timing marks on the seismogram was 5 minutes slow. While your measurements would be very precise, their average value would be roughly 5 minutes different from the true arrival time! The measurements would not be very accurate at all.
The Science Measurement site provides an excellent visual example of the difference between precision and accuracy. The Accuracy and Precision page has a similar example as well as other links related to measurement uncertainty.