A reaction is said to be oscillating if the concentration of one or more intermediates does not vary monotonically, but passes through maximum and minimum values.
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 176 (https://doi.org/10.1351/pac199668010149)