Killer feature
A killer feature or unique selling point is any feature of a product or service that proves so useful as to make it indispensable. Examples include spreadsheets as a feature of the PC, or social networking as a feature of the Internet. Analogous to killer application, which specifically refers to "software as a killer feature" of its compatible hardware (meaning that the software sells the hardware), the term "killer feature" often more specifically refers to software features that sell the software.