killer application

/K IH1 L ER0 AE0 P L AH0 K EY0 SH AH0 N/
noun
  1. 1

    A technological product that is so good, or so innovative, that it is irresistible to potential customers; especially a software product that persuades people to buy the hardware needed to use it.

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Source: https://en.wiktionary.org/wiki/killer%20application