The term 'Unit of Interstellar Distance' refers to the parsec, which is a unit of length used to measure astronomical distances. A parsec is equal to 3.26 light-years, or about 30.9 trillion kilometers. It is derived from the phrase 'parallax of one arcsecond, ' which is the angle subtended by an object at a distance of one astronomical unit (AU). The AU is the average distance between the Earth and the Sun, which is approximately 150 million kilometers.