- showing a response to a stimulus. "pupils are reactive to light"
- acting in response to a situation rather than creating or controlling it."a proactive rather than a reactive approach"
- having a tendency to react chemically."nitrogen dioxide is a highly reactive gas"
A reactionary is a person who holds political viewpoints that favor a return to a previous state in a society. If you live in the U,S. and you want to go back to a time when "women knew their place," you're a reactionary. (Not a conservative: that would mean wanting to keep things from changing, Going backwards is a change.)
Being reactive, on the other hand, is not political. It's a character trait. If you say things like "If it ain't broke, don't fix it" and "Don't borrow trouble," you're probably a reactive person.
"Reactive" can also describe the way one responds to a changing situation. Right now, the U.S. is responding to the crisis in Iraq in a reactive way. In 2003, the U.S. took the initiative. This should tell you that waiting to see is not always the worst policy, and intervention not always the best.
Using "reactionary" when you mean "reactive" confuses the issue. Very progressive people can have knee-jerk responses to things. Right-wingers can plan for years in advance. We need to keep the words that let us say so.