Vector quantity equal to the derivative of the position vector with respect to time (symbols u, v, w for components of c).
Sources:
Green Book, 2nd ed., p. 11 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
ISO 31-1: 1992 (https://www.iso.org/iso/catalogue_detail.htm?csnumber=3623)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 997 (https://doi.org/10.1351/pac199668040957)