What is the definition of prefix plu?

There isn't a prefix "plu" in the English language. It's likely you're thinking of "plu-", which is a prefix meaning "more," "greater," "excess," or "many."

Here are some examples of words using the prefix "plu-":

* Plural: More than one, multiple.

* Plus: Added to, in addition to.

* Plump: Rounded and full, having excess flesh.

* Pluperfect: In grammar, a past perfect tense.

* Plutocracy: Rule by the wealthy.

If you were thinking of a different prefix, please provide more context so I can assist you further.