About a point is the vector product of the radius vector from this point to a point on the line of action of the force and the force, M = r x F.
Sources:
Green Book, 2nd ed., p. 12 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
ISO 31-2: 1992 (https://www.iso.org/iso/catalogue_detail.htm?csnumber=3626)