Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Time and Frequency from A to Z, X to Z


An acronym for a quartz crystal oscillator. It usually refers to the simplest types of quartz oscillators that have no compensation for the effects of temperature.


A year is a time interval whose duration is based on the period of the Earth’s orbit around the Sun.  The Gregorian calendar considers a calendar year to be either a common year of 365 days, or a leap year of 366 days, with the average year having a length of 365.2425 days.  In astronomy, the Julian year is defined as being equal to exactly 365.25 days, with each day having 86,400 seconds.

Zero Beat

The condition reached during a measurement or calibration when the beat frequency between two input signals is no longer detectable. Zero beat is often associated with audio frequency calibrations (such as the tuning of musical instruments), when the person performing the measurement can no longer hear the beat frequency or beat note.


A term sometimes used in the military and in the navigation community as a synonym for Coordinated Universal Time (UTC). In military shorthand, the letter Z follows a time expressed in UTC. Zulu is not an official time scale, nor is it an official designation.  The term originated because the word zulu is the radio transmission articulation for the letter Z, and the time zone located on the prime meridian is designated on many time zone maps by the letter Z.

Created May 12, 2010, Updated March 1, 2023