Reactor is someone who rather than initiate conversation will just react to things said to them, often using one of their set repertoir of one liners. This earns them respect from over hearing third parties and creates the illusion that they are contributing witty banter. This often leaves the initiator looking like the boring person with no way to respond. In reality they are riding the initiator's creativity and steeling the glory. They do not feel social awkwardness and are happy to sit there without engaging or starting conversation. The initiator is forced to keep throwing topics into the mix, each one battered down with one liners leaving the initiator exhausted, bemused and scraping the barrel for conversation while the reactor seems oblivious to their crimes. They have nothing to give and are the scum of society.
Kanye West (see him being a reactor on the Sunday Night Project)
by Adagoof July 15, 2009
A person who often responds or behaves in a particular way in response to something.
Johnny comments on everything I post on social media, he's such a reactor.
by Misheard Metal June 20, 2016