second

of arc

https://doi.org/10.1351/goldbook.S05514

Non-SI unit of plane angle: 1″ = (1/3600)° = 4.84837... x 10E-6 rad.

Sources:
Green Book, 2^nd ed., p. 113 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 991 (https://doi.org/10.1351/pac199668040957)