The angle between two half-lines terminating at the same point is the ratio of the length of the included arc of the circle (with its centre at that point) to the radius of the circle. Symbols: α, β, γ ... This is a quantity of dimension one with the SI unit radian (rad), but very often degrees of arc are used.
Sources:
Green Book, 2nd ed., p. 11 (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 961 (https://doi.org/10.1351/pac199668040957)