This word is used in different senses:
Input of external energy into a chemical system is said to bring about activation of the system.
An added substance that increases the rate of a catalysed reaction is known as an activator, and the effect is called activation. The degree of activation is defined by:
where is the rate of the catalysed reaction in the absence of the activator
and is the rate of the catalysed reaction in the presence of the activator
When some of the energy required for a reaction to occur is provided by a previous exothermic chemical reaction there is said to be chemical activation.
PAC, 1996, 68, 149
(A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996))
on page 151
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook