The "Battered Woman Defense" is an argument used to defend abused woman -- or abused spouses -- from harsh penalties if they are charged with murder or assault (Wikipedia). While it is not a legal defense as such, it is intended to provide a certain amount of leniency depending on the situation, saying in essence that a person was not in their right mind, that the abuse they suffered drove them to commit an act of violence.
The main argument is that a person suffering abuse on a regular basis is entitled to a certain amount of self-defense, if the act is unavoidable under the circumstances. Many people suffering abuse refuse to take legal or other action against their abuser, with guilt and low self-esteem playing a large role. The longer the abuse goes on, the less likely the victim is to seek any sort of authoritative help. With this in mind, a victim who assaults or kills their abuser is understood to have a certain amount of mental instability, which leads to allowances based on circumstance.
In my state, it is a legal defense, as with all affirmitive defenses though, the defendant has the burden of proof to the trier of fact.
Before the Syndrome's adoption, it was a basic concept of criminal law that if one is being abused on a regular basis and does not remove themselves from that environment, they have "assumed the risk" of any potential future harm and can not seriously harm another from one act committed against them.
The enactment of the law does not abrogate that, per se, but provides a defense of being battered as a course of daily life, etc.