Reciprocal of the period.
See also: frequency (in photochemistry)
Source:
Green Book, 2nd ed., p. 11 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
See also:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2244 (https://doi.org/10.1351/pac199668122223)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 974 (https://doi.org/10.1351/pac199668040957)