### Question
How is the term knock defined?


### Answer
The term **knock** is defined in *15 U.S.C. § 2821* as the following:

> the combustion of a fuel spontaneously in localized areas of a cylinder of a spark-ignition engine, instead of the combustion of such fuel progressing from the spark

#### Citation:
  - *15 U.S.C. § 2821*
  - Heading: **CHAPTER 55-PETROLEUM MARKETING PRACTICES > SUBCHAPTER II-OCTANE DISCLOSURE**

