noun /ˈpliːnəm/
The idea was that a thing could only move into an empty place, and that, in a plenum, there are no empty places.
b) A state of fullness, a great quantity (of something).
The key to understanding medieval interpretations of motion in hypothetically void space is to realize that medieval natural philosophers analyzed the same bodies in the void that they discussed in the plenum of their ordinary world.
Wikipedia foundation.