A second of arc or arcsecond is a unit of angular measurement which comprises one-sixtieth of an arcminute, or 1/3600 of a degree of arc or 1/1296000 ≈ 7.7×10-7 of a circle. It is the angular diameter on object of 1 unit diameter at a distance of 360×60×60/2/π ≈ 206,265 units, such as (approximately) 1 cm at 2.1 km, or, directly from the definition, 1 astronomical units at 1 parsec .
Correspondingly, 1 radian ≈ 206,265 arcseconds.
The symbol for marking the arcsecond is the double prime (″) (U+2033, ″). One arcsecond would be 1″ (or 1").
It can be abbreviated as arcsec, but should then not be confused with the inverse trigonometric function arc secant, which has the same abbreviation.