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.
Source:
J T Morgan, ‘An Historical Analysis of the Principle of Double Effect’, Theological Studies (1949)
The doctrine (or principle) of double effect is often invoked to explain the permissibility of an action that causes a serious harm, such as the death of a human being, as a side effect of promoting some good end. According to the principle of double effect, sometimes it is permissible to cause a harm as a side effect (or “double effect”) of bringing about a good result even though it would not be permissible to cause such a harm as a means to bringing about the same good end.