The "black/death metal" take on the word "vocals". A form of vocalization used in extreme genres of heavy metal, ranging from a low unintelligible growl in death metal to a raspy shrieking found in black metal. Variations of black and death metal, such as melodic death metal also use this form of lyrical vocalization (one can't really label it as "singing" because of its nature).
Vokills are not to be confused with simply shouting or screaming into the microphone; the utterances have an otherwordly quality, being used for meaningful invocation and representation of atmosphere rather than just "shock factor".
Abbath's vokills really rocked on Immortal's "Sons of Northern Darkness"!