Adjective

(en adjective)

Opposed to change; urging a return to a previous state.

Very conservative.

* {{quote-book

, date = 2011-09-29 , title = The Reactionary Mind: Conservatism from Edmund Burke to Sarah Palin , author = Corey Robin , location = New York , publisher = Oxford University Press , isbn = 9780199793747 , ol = 25101368M , page = 25 , pageurl = http://books.google.com/books?id=CWxIz_Gm9WMC&pg=PA25&dq=reactionary , passage = There's a fairly simple reason for the embrace of radicalism on the right, and it has to do with the reactionary imperative that lies at the core of conservative doctrine. If he is to preserve what he values, the conservative must declare war against the culture as it is. }}

(chemistry) Of, pertaining to, participating in or inducing a chemical reaction.

* 2013 , Brandon Smith, Are Individuals The Property Of The Collective?

Psychiatry extends the theory into biology in the belief that all human behavior is nothing more than a series of reactionary chemical processes in the brain that determine pre-coded genetic responses built up from the conditioning of one’s environment.

Adjective

(wikipedia undefined) (-)

Lacking a definition or value.

(mathematics, computing) That does not have a meaning and is thus not assigned an interpretation.

The result of division by zero is undefined .