Heat brought to a system to increase its temperature divided by that temperature increase. At constant volume CV = (∂U/∂T)V at constant pressure Cp = (∂H/∂T)p, where U is the internal energy and H the enthalpy of the system.
Green Book, 2nd ed., p. 48 (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 975 (https://doi.org/10.1351/pac199668040957)