In marketing terminology, a killer feature is any attribute of a product that proves so useful as to become the main attraction of the product. Examples include spreadsheets as a feature of the PC, or pornography 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.
References
Killer feature Wikipedia(Text) CC BY-SA