In simple hard sphere collision theory the area of the circle with radius equal to the collision diameter.
Source:
Green Book, 2nd ed., p. 56 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
See also:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 158 (https://doi.org/10.1351/pac199668010149)