Definition of plenist in US English:

plenist

noun

historical
  • An adherent of the theory that all space is full of matter and that there is no such thing as a vacuum.

Origin

Mid 17th century; earliest use found in Robert Boyle (1627–1691), natural philosopher. From classical Latin plēnum plenum + -ist. Opposed to vacuist.

Pronunciation

plenist

/ˈpliːnɪst/