What is a prefix for graduate?

There isn't a single, universally accepted prefix for "graduate." However, depending on the context, you could use:

* Post- (e.g., postgraduate studies, post-graduate job)

* Former (e.g., former graduate student)

* Alumni (e.g., alumni association)

* Ex- (e.g., ex-graduate student)

You can also use phrases like:

* A graduate of... (e.g., a graduate of Harvard University)

* Someone who has graduated... (e.g., someone who has graduated with a degree in engineering)

The best prefix or phrase to use depends on the specific situation.