What is the definition of polyhedron?

Again we go back to ancient Greek.

This should not surprise you since the father who set the basis of Geometry was the Greek: Εὐκλείδης, (Euclid) who lived between the third and second century BC in Alexandria.

The key word here is πολύεδρον, pronounced polyhedron and meaning "of many bases"

πολύς (pronounced polis) means many ἕδρα, (pronounced edra) means basis

A polyhedron therefore is a three dimensional solid with a polygon of any number f faces as a base