Focus on the pronunciation of killer app
English definition of “killer app”
noun [C] (also killer ap, killer application) IT
› 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.