What is prefix for decisive?

There isn't a specific prefix that directly means "decisive." However, you can use prefixes to create words with a related meaning:

* Deci- means "tenth" or "ten" (e.g., decimeter) and is not directly related to "decisive."

* Pre- means "before" or "prior to" (e.g., predetermine, preempt). You could use this prefix to describe something that happens *before* a decisive moment.

* In- means "in" or "into" (e.g., in-depth) and can be used to describe something that is *thoroughly* decisive.

Ultimately, the best prefix will depend on the specific context and the nuance you want to convey.