killer app

noun [C] (also killer ap, killer application) IT

Definition

› computer software that is very popular or useful, especially software that is the reason people buy certain computer systems, mobile phones, etc.: Mobile access to social networking sites is the next killer app.

› a use for a particular technology that becomes extremely popular and is the reason people want that technology: At the time they thought that e-learning would be the killer app of the Internet.

(Definition of killer app noun from the Cambridge Business English Dictionary © Cambridge University Press)