killer

Killer is a term that functions as both a noun and an adjective in English. As a noun, it denotes a person who kills someone, typically a murderer. As an adjective, killer means deadly or extremely effective or impressive, sometimes used colloquially. The word is also used in various specialized senses in science and culture.

In criminology, killers are individuals who intentionally cause the death of another person. Subcategories include serial killers (unlawful killings over a period with cooling-off intervals), mass killers (several in a single event), spree killers (several in a short period across locations), and contract killers (hits performed for payment).

In biology, killer refers to natural killer cells, a type of lymphocyte that targets virus-infected or tumor cells as part of the innate immune response. They identify stressed cells and induce apoptosis through cytotoxic granules.

In the animal world, killer whale, or orca, is a large predatory marine mammal known for its complex social structure and apex predation. The term comes from its reputation as a top predator rather than implying that it kills alone.

In culture, killer is used in titles and phrases, such as “killer app” describing a product that drives widespread adoption, or as a descriptor in music, film, and literature. The exact connotation depends on context, ranging from neutral to pejorative.

See also killer (disambiguation).