Arcsec, sometimes written as "arcsecond," is a unit of angular measurement used in astronomy and related fields. It represents one-sixtieth of an arcminute, or one-three-thousand-six-hundredth of a degree. Synonyms for arcsec include "second of arc" and "arc second." Other related terms include "mas" or "milli-arcsecond," which represent one-thousandth of an arcsecond, and "microradian," which is roughly equivalent to 206,265 arcseconds. Arcseconds are used to describe the size of celestial objects, such as stars or planets, and the angular separation between them. They are also used to measure the accuracy and resolving power of telescopes and other instruments used in astronomy.