manslaughter

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The killing of a person without malice aforethought but with either the intention to commit an unlawful act that leads to an unintended death, or with an otherwise murderous intent that is extenuated by some partial defense, such as acting under the influence of an extreme emotional disturbance occasioned by a substantial provocation on the part of the victim.

from The Century Dictionary.

• noun The killing of a human being by a human being, or of men by men; homicide; human slaughter.
• noun Specifically—2. In law, the unlawful killing of another without malice either express or implied, which may be either voluntarily, upon a sudden heat, or involuntarily, but in the commission of some unlawful act. Blackstone.

from the GNU version of the Collaborative International Dictionary of English.

• noun The slaying of a human being; destruction of men.
• noun (Law) The unlawful killing of a man, either in negligence or incidentally to the commission of some unlawful act, but without specific malice, or upon a sudden excitement of anger.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An act of killing a human being unlawfully, but not wilfully (as opposed to murder).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun homicide without malice aforethought