It's commonly known as "Battered Woman Syndrome." As Smurfette said, it is used as an "imperfect defense" to murder in an attempt to mitigate murder down to manslaughter. The idea is that self-defense can be used in a situation where a "rational" person would believe they are at risk of death or serious bodily injury. A person who has been systematically abused by their spouse can reach a point where, even though they are not at that exact moment being attacked, they believe they must kill their partner or they will be killed. So while (s)he honestly believes (s)he will die unless (s)he kills his/her partner at that moment, most people would not consider that belief "rational." (Because the partner was asleep, watching TV, etc.)
49 states* and DC allow expert testimony regarding BWS, though not all states allow the expert to testify about whether the defendant in that exact case suffered from BWS.
* North Carolina is the exception.
(I am taking a Domestic Violence class this semester. And I am doing my paper on male victims of DV, and so am a little touchy about assuming victims are female.
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