means A vessel or device designed to sustain and control a reaction — most often a nuclear chain reaction, but also a chamber for chemical processes — so its energy can be harnessed rather than unleashed.

from From Latin 're-' (back, again) plus 'agere' (to do, drive, set in motion) — the same root that gives us 'act' and 'agent.' To 'react' is literally to do back, to answer one motion with another. 'Reactor' adds the agent-suffix '-or,' naming the thing that does the reacting. The chemical sense came first; the nuclear 'reactor' arrived with the atomic age in the mid-20th century, when physicists needed a word for the box where atoms answer each other in a chain.