fatal, deadly, mortal, lethal. These adjectives apply to what causes or is likely to cause death. Fatal describes conditions, circumstances, or events that have caused or are destined to cause death or dire consequences: a fatal illness, a fatal error. Deadly means capable of killing: a deadly poison. Mortal describes a condition or action that produces death: a mortal wound, mortal combat. Lethal refers to a sure agent of death that may have been created solely for the purpose of killing: execution by lethal injection.
edited from http://www.answers.com/topic/fatal

...wow, I'm not a regular here, but this post sure helped my work this night! Thank you so much for those definitions!

Wow, laalaa! - Laß mich das, zwecks besserer Übersicht für alle, strukturieren:

fatal, deadly, mortal, lethal. These adjectives apply to what causes (or is likely to cause) death.

fatal describes conditions, circumstances, or events that have caused or are destined to cause death or dire consequences: a fatal illness, a fatal error.

deadly means capable of killing: a deadly poison.

mortal describes a condition or action that produces death: a mortal wound, mortal combat.

lethal refers to a sure agent of death that may have been created solely for the purpose of killing: execution by lethal injection.
. . .