Non-SI unit of energy. There are three differently defined calories: thermochemical calorie
(calth = 4.184 J), international steam table calorie
(calIT = 4.1868 J), 15°C calorie
(cal15 = 4.1855 J), however only the symbol cal is usually used.
Sources:
Green Book, 2nd ed., p. 112 (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 964 (https://doi.org/10.1351/pac199668040957)