- An extremely accurate reference clock whose operation is based on an atomic process, typically the frequency of electromagnetic radiation associated with a specified energy-level transition in an element such as cesium.
- (proscribed) A radio clock.
- The U.S. National Institute of Standards and Technology, which operates the WWVB time broadcast, advises that referring to a clock not containing its own atomic oscillator as an "atomic clock" is technically incorrect and should be avoided.
- ^ Michael A. Lombardi et al. (2009), “Use of "Atomic Clock" Nomenclature”, in WWVB Radio Controlled Clocks: Recommended Practices for Manufacturers and Consumers (NIST Special Publication 960-14), page 34:
Many WWVB RCC products are labeled (on the product itself or in the documentation) as "atomic clocks." This is probably seen by manufacturers as a useful marketing tool intended to capture the imagination of potential customers, and some might argue that it is appropriate since atomic clocks are located at the WWVB radio transmitter site. However, we contend that use of the term "atomic clock" is technically incorrect and misleading to consumers, and its usage should be avoided. Unless there is actually an atomic oscillator inside the RCC (such as a cesium or rubidium oscillator), we recommend that the term "radio controlled clock" be used to correctly describe the product. Labeling products or documentation with the term "atomic timekeeping" is also considered acceptable.