The term 'Interstellar Unit' is a unit of measurement used to measure distances between stars. It is equal to 3.26 light-years, and is abbreviated as 'ISU'. The term is also used in astronomy to describe the distance between two stars. The term is derived from the word 'parsec', which is a unit of length equal to 3.26 light-years. A parsec is the distance at which one astronomical unit subtends an angle of one arcsecond.