Crystals are one of the stable signal generators, now atomic clock is the highly stable in recent times.
How ever crystal can be tuned by temperature, voltage etc.
TCXO --> Temperature compensated crystal oscillator
VCXO --> Voltage controlled crystal oscillator
OCXO--->Owen controlled crystal oscillator
PPM--> parts per million
Get the additional details from below links
https://www.wenzel.com/documents/tcxo.html
https://www.vectron.com/products/appnotes/index.htm
**broken link removed**