plenum
n.a meeting where everyone in a group is present. It can also mean a space that is completely full of something, like air or gas.
n.a meeting of an entire body or assembly; also, a chamber or space filled with a fluid or gas. Often used in technical contexts to describe a volume of air or a full assembly of delegates.
“The committee will discuss the proposal in a plenum tomorrow.”
“After the breakout sessions, all the delegates gathered in the main hall for a plenum to vote on the final report.”
“The physicist explained that the experiment required a vacuum, as the presence of air in the plenum would interfere with the delicate measurements of the particle's trajectory.”
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.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.