|This article is about an older version of DF.|
A kill is attributed to a dwarf when they deal the final killing blow to anything they are fighting. It's stored in a dwarf's kill list, which details the name (if applicable), creature type (e.g. kobold, lion, etc.), and date of the kill. You can see the battle history of a dwarf by looking them up with -- or --. If they do not have one, the option will not be available. Slaughtering an animal does not count as a kill.
It does not necessarily have to be a hostile creature; fortresses that get invaded by ravens may have several of their dwarves gain kill lists early on. Most dwarves lack any previous history of fighting when they immigrate to your fortress, however, a second wave potash maker may eventually gain a rather extensive list of murdered goblins during his time in your fortress. Note that vampires will arrive at your fortress in disguise, and will not have their (usually extensive) kill list attached. You can still see how old and damned they are by looking them up in legends mode, once you figure out their true name.
Weapons (and shields) have kill lists too; examine the weapons (-- on a dwarf's person or - otherwise) and then press to see its kill list. Weapons with a long kill list will eventually be promoted to semi-legendary status and appear on the egendary artifacts screen, albeit in blue.
Dwarves who kill enough "notable" enemies will have special titles added to their names, to the effect of "Urist McArcher the Leaves of Vengeance" (or "Cacame Apebalded the Immortal Onslaught"). This is independent of their being elite or not.