means a response to something — whether a chemical change, a physical force, an emotional reflex, or a political pushback against progress.

from From Latin 're-' (back, again) plus 'actio' (a doing, a driving), built on 'agere,' to do or set in motion — a cousin of words like 'agent' and 'act.' So a reaction is literally a 'doing-back,' a response shoved into motion by something that moved first. The word entered English through scientific and philosophical Latin, and only later did it acquire its political sting, where a 'reactionary' is someone driving the world backward against the push of change.