What is the definition of prefix fall?

There is no standard definition for a "prefix fall." It's not a recognized term in linguistics or computer science.

Could you please provide more context or explain what you mean by "prefix fall"?

For example, are you referring to:

* A decline in the use of prefixes in a language?

* A specific phenomenon in computer science related to prefixes?

* Something else entirely?

Knowing what you're looking for will help me give you a more accurate answer.