What is the "rule of double effect?"

It's an ethical principle which states that effects that would be morally wrong if caused intentionally are permissible if foreseen but unintended. This principle is often cited to explain why certain forms of care at the end of life, such as administering high-dose opioids, are morally permissible. The rule does not permit practices such as physician-assisted suicide.

The rule of double effect emphasizes four key conditions:

1. the nature of the act must be good or at least morally neutral
2. the good effect and not the evil effect must be intended
3. the bad effect, such as death, must not be a means to the good effect, such as relief of suffering
4. the good effect must outweigh the bad effect
   

Source

Quill, T. and Dresser, R. "The Rule of Double Effect - A Critique of Its Role in End-of-Life Decision Making" NEJM. 11 Dec 1997.