plenum

The story

1. Russian · semantic loan fromпле́нум“plenary session”
2. Latin · derived fromplēnum n“space filled with matter”
3. English todayplenum

From Latin plēnum n (“space filled with matter”), from plēnus (“full”). The sense of "legislative meeting" is a semantic loan from Russian пле́нум (plénum, “plenary session”), from the same Latin source.