Ethical doctrine, associated especially though not exclusively with Roman Catholicism.
Though we may not intentionally produce evil, we may intentionally do (in pursuit of a suitably greater good) what we foresee will in fact produce evil, provided we regard this evil as an unwanted side-effect which we would avoid if possible.
The occurrence of the evil must not be necessary as a means to our intended end.
The doctrine might, for example, forbid us to torture an innocent person to gain vital information in a good cause, while allowing us to kill an innocent civilian while bombing a munitions depot, provided that the intended good greatly outweighs the expected evil.
J T Morgan, 'An Historical Analysis of the Principle of Double Effect', Theological Studies (1949)