Princeton's WordNet

1. killer, slayer(noun)

   someone who causes the death of a person or animal

2. cause of death, killer(noun)

   the causal agent resulting in death

   "heart disease is the biggest killer in the United States"

3. killer(noun)

   a difficulty that is hard to deal with

   "that exam was a real killer"

4. killer whale, killer, orca, grampus, sea wolf, Orcinus orca(noun)

   predatory black-and-white toothed whale with large dorsal fin; common in cold seas

Kernerman English Learner's Dictionary

1. killer(noun)ˈkɪl ər

   sb or sth that causes sb to die

   Her killer has never been caught.; The disease is a killer.

Wiktionary

1. killer(Noun)

   That which kills.

2. killer(Noun)

   That which causes stress or is extremely difficult, especially that which may cause failure at a task.

3. killer(Noun)

   Something that is so far ahead of its competition that it effectively kills off that competition.

   Various means had were used to steer aircraft in the early years but ailerons were the killer.

4. killer(Noun)

   A knockout form of darts involving several players

5. killer(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).

6. killer(Adjective)

   Excellent, very good.

Webster Dictionary

1. Killer(noun)

   one who deprives of life; one who, or that which, kills

2. Killer(noun)

   a voracious, toothed whale of the genus Orca, of which several species are known

• moordenaarAfrikaans
• قاتِلArabic
• убиец, убийственBulgarian
• assassinoPortuguese (BR)
• zabiják; smrtícíCzech
• der Mörder; Mörder-German
• morder; dræber-Danish
• δολοφόνοςGreek
• asesinoSpanish
• tapjaEstonian
• کشنده؛ قاتلFarsi
• tappajaFinnish
• tueur/-euseFrench
• רוֹצֵחַHebrew
• हत्याराHindi
• ubojicaCroatian
• gyilkosHungarian
• pembunuhIndonesian
• morðingiIcelandic
• killer, assassino, uccisoreItalian
• 殺人者Japanese
• 죽이는 사람Korean
• žudikas; mirtinasLithuanian
• slepkavaLatvian
• pembunuhMalay
• moordenaar; dodelijkDutch
• drapsmann, morder, dreperNorwegian
• morderca, morderczyPolish
• کشنده؛ قاتلPersian
• قتل كونكې،وژونكېPashto
• assassinoPortuguese
• ucigaşRomanian
• убийцаRussian
• zabijak; kto zabíja dobytok; smrteľnýSlovak
• ubijalecSlovenian
• ubicaSerbian
• dödare, mördare, dråpareSwedish
• ฆาตกรThai
• katilTurkish
• 殺人者Chinese (Trad.)
• убивця, кілерUkrainian
• قاتل ، خونیUrdu
• kẻ giết ngườiVietnamese
• 杀人者Chinese (Simp.)