Terrestrial Dynamical Time (TDT) is a scientific term used in the field of astronomy. It refers to the precise timekeeping system used to measure the movements of objects in space relative to the Earth. Synonyms for TDT include Terrestrial Time (TT), Geocentric Coordinate Time (TCG), and Earth Dynamic Time (EDT). These terms are often used interchangeably, but they have subtle differences in their definitions and uses. While TDT and TT are primarily used for astronomical applications, TCG and EDT are used more broadly in geophysics and other scientific fields. Regardless of which term is used, all of them refer to the same fundamental concept of measuring time in relation to the Earth's rotation and orbit.