killer

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

• noun One that kills.
• noun Slang Something that is extremely difficult to deal with or withstand.
• adjective Causing death or destruction.
• adjective Having impressive or effective power or impact; formidable.
• adjective Extremely useful or indispensible.

from The Century Dictionary.

• noun A contrivance for killing large ferocious animals.
• noun The technical name among house-painters for anything used to prevent resin, locally present in woodwork, from exuding and making visible spots on the painted surface; also, in a more general sense, any substance used to remove spots on such surfaces.
• noun One who kills or deprives of life; especially, a slaughterer; a butcher.
• noun A club of hard wood, used for killing fish.—
• noun A delphinid, Orca gladiator, and other species of that genus: so called from their ravenous and ferocious habits.

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

• noun One who deprives of life; one who, or that which, kills.
• noun (Zoöl.) A voracious, toothed whale of the genus Orca, of which several species are known; called also killer whale.

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

• noun That which kills.
• noun figuratively That which causes stress or is extremely difficult, especially that which may cause failure at a task.
• noun figuratively Something that is so far ahead of its competition that it effectively kills off that competition.
• noun sports A knockout form of darts involving several players.
• noun A diacritic mark used in Indic scripts to suppress an inherent vowel (e.g., the Hindi viram, the Bengali or Oriya hasanta) or render the entire syllable silent (e.g., the Burmese virama, the Khmer toandakhiat).
• adjective slang Excellent, very good.

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

• noun predatory black-and-white toothed whale with large dorsal fin; common in cold seas
• noun a difficulty that is hard to deal with
• noun someone who causes the death of a person or animal
• noun the causal agent resulting in death