Precision vs. accuracy
Imagine an archer who has shot ten arrows. In this scenario, precision is a measurement of the arrows’ positions relative to each other and accuracy is a measurement of their positions relative to the bullseye. A precise archer isn’t necessarily an accurate one, and vice versa.
The precision of an archer is analogous to a concept called clock stability. If one thinks of each tick of the clock as a shot and hitting the bullseye as keeping the exact right time between every tick, then a precise but not accurate clock would consistently tick either slower or faster than the desired amount of time. On the other hand, an accurate but imprecise clock would tick sometimes faster and sometimes slower, but the accumulated errors would average out somewhat over time.
Via Inside Science: Why Do We Need Super Accurate Atomic Clocks?