Stability describes how well the clock frequency resists fluctuations. Factors that can cause the frequency to fluctuate include variations in temperature, time (aging), supply voltage, shock, vibration, and capacitive load that the clock must drive. Temperature is often the dominant factor that affects crystal oscillator stability.
Some oscillators are housed inside small ovens with controlled temperature to provide stability that can be orders of magnitude better than with other techniques. These oscillators are known as oven controlled crystal oscillators (OCXOs). For example, the NI 6608 contains an OCXO.