It is not merely "one who reacts". I believe it's origins are in Marxist jargon, much of which was translated from German. In Cuban revolutionary argot, they use "contra-revolucionario", instead of "reaccionario". The word "react" did not once mean "respond", it was used more like Newton's "equal and opposite reaction", to push back.

I've always understood it to mean someone who "wants to go back to the way things used to be", presumably, in the good old days. Of course, they are the ones who decide when the good old days were, and why they were so good. It is always used perjoratively, and only by their opponents. Reactionaries usually call themselves traditionalists--or patriots.

Political definitions are fluid and tell you more about the people who coin them than the people they describe. For example, "the bourgeosie" is a French word meaning "the townfolk", in Spanish, "la burgesia" means small businessmen, bureaucrats, government employees, minor professionals, schoolteachers, and managers, etc. It has no particular political connotation or significance. Among Marxists it refers to the property-owning class that controls capital and the means of production. You can't look up these things in Webster's and pin them down precisely, and anyone who tells you you can is trying to define the terms of debate in his favor. Don't let them get away with it.

Incidentally, Americans have this thing about dictionaries. They look something up and they think they understand what it means. Language is a living thing, it is defined by common usage, not by scholarship. A good dictionary does not define what a word is supposed to mean, it tells you how it is used in conversation and literature, both contemporarily, and historically. Look at how the meaning of "Liberal" has changed since the 18th century. This is not a perversion of the language, it is a perfectly normal linguistic drift. In Shakespeare, many words have different meanings than they do today.

I was originally taught the political spectrum, from R to L, was Reactionary--Conservative--Moderate--Liberal--Radical. But there is no such thing as a "correct" definition, there are only terms of convenience.