I am trying to understand the nuances of using the adjective affirmative in the context of affirmative defense. How is an affirmative defense different from a defense (sans any adjective modifier)?
Generally an affirmative defense raises some ground other than an element of the offense or civil claim, such as justification (e.g. self-defense), privilege (I'm a soldier acting under lawful orders so I'm immune from liability), the invalidity of a law, statute of limitations, insanity, a pardon, bankruptcy, a settlement agreement, or payment of the amount owed in full. In a civil case, an affirmative defense has to be raised in the answer to the complaint failed by the plaintiff or a timely amendment to that answer.
The prosecution/plaintiff has to overcome an affirmative defense only if the defendant raises it, and in a civil case, at least, the burden of proof for an affirmative defense is on the defendant. The requirement that a defendant raise the defense going to something beyond the elements of the crime or civil claim is what makes it an "affirmative" defense. Affirmative defenses are conclusively presumed to be unavailable to the defendant if not raised.
Usually, it is possible to prevail in a civil or criminal case by establishing a complete affirmative defense, even if the defendant admits all of the elements of the crime or civil claim against the defendant have been proven.
Also, many affirmative defenses (e.g. failure to mitigate damages, or heat of passion) are incomplete affirmative defenses that reduce the consequences that a defendant will face if the prosecution/plaintiff establishes a prima facie case (i.e. proves all the elements of the crime or civil claim), but will not completely eliminate all consequences of the prosecution/plaintiffs' proof of all elements of the crime or civil claim for the defendant.
But the prosecution/plaintiff must always prove each and every element of a crime or civil claim (beyond a reasonable doubt in criminal cases, and by preponderance of the evidence or by clear and convincing evidence in most non-criminal claims) even if the defendant fails to present any evidence or make any arguments related to the element of the crime or civil claim to prevail on the merits.
Something that undermines an element of the crime or civil claim is a mere regular defense, because it doesn't have to be raised affirmatively, although in civil cases, facts that are alleged in a plaintiff's complaint are deemed admitted if they are not affirmatively denied, making the distinction between affirmative defenses and ordinary defenses in civil cases rather subtle.
Of course, the term "defense" can be used to encompass both affirmative defenses, and other defenses which are not affirmative defenses (such as defenses that disprove an element of the crime or civil claim).
Just to complicate matters further, there are a small number of ordinary defenses that still have to be raised affirmatively in advance in a criminal case despite the fact that it disproves an element of the crime, rather than advancing some other matter. The main example of this is an alibi defense (i.e. I wasn't at the scene of the crime when it happened).
You are in court defending yourself because someone claims you did X and you should be punished for it or pay damages.
A defense is you saying “I didn’t do X”. An affirmative defence is either you saying “Even if I did X, I was allowed to do it”, or “Even if I did X and even if I wasn’t allowed to do it, I shouldn’t be punished for it”.
In civil proceedings and criminal prosecutions under the common law, a defendant may raise a defense (or defence) in an attempt to avoid criminal or civil liability. Besides contesting the accuracy of any allegation made against them in a criminal or civil proceeding, a defendant may also make allegations against the prosecutor or plaintiff or raise a defense, arguing that, even if the allegations against the defendant are true, the defendant is nevertheless not liable.
An affirmative defense to a civil lawsuit or criminal charge is a fact or set of facts other than those alleged by the plaintiff or prosecutor which, if proven by the defendant, defeats or mitigates the legal consequences of the defendant's otherwise unlawful conduct. In civil lawsuits, affirmative defenses include the statute of limitations, the statute of frauds, waiver ... examples of affirmative defenses are self defense, insanity, and the statute of limitations.
In an affirmative defense, the defendant may concede that they committed the alleged acts, but they prove other facts which, under the law, either justify or excuse their otherwise wrongful actions, or otherwise overcomes the plaintiff's claim. In criminal law, an affirmative defense is sometimes called a justification or excuse defense. Consequently, affirmative defenses limit or excuse a defendant's criminal culpability or civil liability.
An affirmative defense is a defense which will counteract one element of a criminal or civil charge, but not the charge itself, while the standard defense or a negating defense will deign the evidence in support of the charge. For a criminal example, 1st degree Murder is usually defined as "killing of another human (homicide) with malicious afterthought" for the purposes of this scenario.
Alice is charged with 1st degree murder for killing Bob with her red car, and then speeding off into the night. Witness at the scene said they saw Alice speed up as Bob moved into her path.
In a traditional defense, Alice might provide evidence that would negate the witness testimony (i.e. "My break lines were cut", or "I own a blue car", or "I wasn't in state that night." or "Charlie borrowed my car that night."). These would work to poke holes in the prosecution's story and serve to create doubt that the witnesses were describing a crime.
However, if Alice's is an affirmative defense, Alice is basically saying "Yes, I killed Bob (homicide) but I didn't do it with Malice Afterthought." Since both elements must be true to be the crime of Murder, Alice must disprove it she had Malice Afterthought at the time (i.e. "Bob was pointing a gun at me, I killed him in self-defense", or "I am crazy and didn't know what I was doing at the time."). In this case, Alice is shifting the burden of proof onto herself, and depending on which defense she uses, it might work. (The self-defense argument is a lot more likely, as insanity typically hinges on if you took steps to hide your crime. Her driving away at high speed looks like she's either trying to get away with her crime, invalidating insanity, or was so frightened as a possible victim she didn't look back to see if Bob was okay. Hit and run is covered in this same defense.).