Origin: Latin suffix -ive
Reactive has 3 different meanings across 1 category:
participating readily in reactions
"sodium is a reactive metal"
"free radicals are very reactive"
reacting to a stimulus
"the skin of old persons is less reactive than that of younger persons"
that reacts or responds to a stimulus
"The immune system mounts a defensive response when it encounters an antigen, making it highly reactive to foreign invaders."
In plain English: Reactive means responding quickly to something that just happened instead of planning ahead.
"The team had to wait for a reactive response from management before they could move forward with their plan."
The word reactive comes from combining the verb react with the suffix -ive to describe something that responds or acts back. It entered English as a straightforward formation based on the existing concept of reacting.