What is the definition of prefix act?

There's no established grammatical term or concept called "prefix act." Prefixes are morphemes (meaningful units) added to the *beginning* of a word to modify its meaning. They don't "act" in a way that's defined as a separate grammatical concept.

Perhaps you're thinking of something else? Could you clarify what you mean by "prefix act"? For example, are you thinking about:

* The effect a prefix has on a word's meaning? Prefixes *change* a word's meaning, but this is a description of their function, not an "act."

* A specific prefix in a particular context? If you can provide an example, it would be easier to understand what you mean.

* Something else entirely? Perhaps you're using the term in a non-standard or specialized way.