Crystal oscillator

In telecommunication, a crystal oscillator (XO) is an oscillator in which the frequency is controlled by a piezoelectric crystal.

Note 1: A crystal oscillator may require controlled temperature because its operating frequency is a function of temperature.

Note 2: Types of crystal oscillators include voltage-controlled crystal oscillators (VCXO), temperature-compensated crystal oscillators (TCXO), oven-controlled crystal oscillators (OCXO), temperature-compensated-voltage controlled crystal oscillators (TCVCXO), oven-controlled voltage-controlled crystal oscillators (OCVCXO), microcomputer-compensated crystal oscillators (MCXO), and rubidium crystal oscillators (RbXO).

Source: from Federal Standard 1037C