Wikipedia - Angolo solido solid angle, $$\varOmega$$, $$\omega$$

https://doi.org/10.1351/goldbook.S05732
Of a cone, the ratio of the area cut out on a spherical surface (with its centre at the @A00412@ of that cone) to the square of the radius of the sphere. It is a quantity of dimension one (dimensionless quantity) with unit @S05971@ ($$\mathrm{sr}=1$$).
Sources:
Green Book, 2nd ed., p. 11 [Terms] [Book]
PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 992 [Terms] [Paper]