Full Idea
'Double effect' refers to action having an effect aimed at, and also one foreseen but in now way desired. The 'doctrine' is that it is sometimes permissible to bring about by oblique intention what one may not directly intend.
Gist of Idea
A 'double effect' is a foreseen but not desired side-effect, which may be forgivable
Source
Philippa Foot (Abortion and the Doctrine of Double Effect [1967], p.20)
Book Reference
Foot,Philippa: 'Virtues and Vices' [Blackwell 1981], p.20
A Reaction
Presumably this can only be justified by a trade-off. The unfortunate side effect must be rated as a price worth paying. If the side effect is not foreseen, that is presumably either understandable, or wickedly negligent. No clear rule is possible.
Related Idea
Idea 22385 The doctrine of double effect can excuse an outcome because it wasn't directly intended [Foot]