The duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. The definition was added to the International System (SI) of units in 1967.

The stability of a time or frequency signal over a short measurement interval, usually of 100 seconds or less.

An astronomical time scale that is based on the Earth's rate of rotation measured relative to the fixed stars. Thus a sidereal day is the time interval during which the Earth completes one rotation on its axis and some chosen star appears to transit twice consecutively on the observer's local meridian. Because the Earth moves in its orbit about the Sun, a mean solar day is about 4 minutes longer than a sidereal day. Thus, a given star appears to rise 4 minutes earlier each night, relative to solar time, and different stars are visible at different times of the year.

A radio wave that bounces off the ionosphere and returns back to Earth. Skywave propagation is a characteristic of HF radio signals, such as those transmitted by WWV. Since the path delay of a skywave signal is constantly changing, skywaves are not as suitable for time and frequency measurements as groundwave or satellite signals.

The day defined as one revolution of the Earth on its axis with respect to the Sun. Since the Earth's rotational period (one day) rate is much more variable than its period of revolution about the Sun (one year), the mean solar day is more useful for timekeeping, since it averages the length of the day over the course of a year.

An astronomical time scale that is based on the Earth's rate of rotation, measured with respect to either the "ficticious" mean Sun, or of the "true" apparent Sun. In the apparent solar time scale, noon is the instant when the Sun transits, i.e., crosses the local meridian and reaches its highest point in the sky, called Local Apparent Noon (LAN). Over the course of the year, the Sun has considerably different daily high points, lowest in the winter and highest in the summer, due to the tilt in the Earth's axis of about 23.44 degrees. The daily variation, over the course of a year, in the Sun's apparent angular speed across the sky causes LAN to occur at a slightly different time each day. The variance in the apparent angular speed is due to a daily change in the Earth's distance from the Sun. In the late fall and early winter months, when the Earth is closer to the Sun (near or at perihelion), the Sun appears to travel faster (a greater angle per unit of time), than during the late spring and early summer months, when the Sun is farther away (near or at aphelion).

The difference between apparent and mean solar time is known as the "equation of time" and is a measure of the apparent Sun preceding or following the mean Sun by an interval than can be as much as 16 minutes. Therefore, mean solar time (based on the length of an average day) is more useful for uniform timekeeping than apparent solar time. In addition, since the Earth is much closer to the Sun than to other stars, one complete rotation of the Earth relative to the Sun (mean solar day) requires about four more minutes than one sideral day.