A reactionary is often described as a conservative, someone that wants to protect some status quo or tradition against change. This is inadequate and we already have a name for that (conservatism). A reactionary goes beyond this and pushed violently against left action even if they are not trying to preserve or conserve a way of being, policy, tradition, etc. Nazis were reactionary, a response to the left that used some mores of tradition but really sought a resolution to the crises or capitalism for part of the ruling class, scapegoating and sacrificing the rest. They did not try to conserve overall, they sought a "third way" that maintained capitalism, capturing both anti-left sentiment and populist sentiment.

So, the defining aspect of reaction is that they are a response to the left, and push back on it hard, regardless of how close their own goals hew to the status quo. A reactionary might want to nuke half the planet if it means the end of communism. A reactionary justifies all violence done to socialists and joins their efforts to it, even if it's a fundamental violation big what they claim to hold dear (e.g. rule of law). A reactionary is the vicious chud who misogynistically attacks AOC because she is perceived as a left icon and the best he can do is chuckle at "AOC is stupid" memes.