The millihertz (mHz) is a unit of frequency that measures the number of cycles or oscillations per second in a system, representing one-thousandth of a hertz (Hz). It is denoted by the symbol mHz and is commonly used in various scientific and engineering applications where low-frequency phenomena are studied. In fields such as seismology, the millihertz range is crucial for analyzing seismic waves and understanding the Earth's internal structure. Additionally, in power systems and electrical engineering, the millihertz unit helps in specifying and analyzing the frequency stability of power grids. The millihertz's precision in measuring low frequencies makes it an essential tool in both practical and scientific contexts.