second

of arc
https://doi.org/10.1351/goldbook.S05514
Non-SI unit of @P04686@: \(1^{\prime\prime} = \frac{1}{3600}\ ^{\unicode{x26ac}} = 4.848\ 137\,...\,\times\ 10^{-6}\ \text{rad}\).
Sources:
Green Book, 2nd ed., p. 113 [Terms] [Book]
PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 991 [Terms] [Paper]