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plenum --Merriam-Webster
noun
ple·num ˈple-nəm
ˈplē-
1
a
: a space or all space every part of which is full of matter
b
: an air-filled space in a structure
especially : one that receives air from a blower for distribution (as in a ventilation system)
2
: a general assembly of all members especially of a legislative body
3
: the quality or state of being full
Etymology
borrowed from Latin plēnum, noun derivative from neuter of plēnus "full"; (sense 1a) after its use in the Lucullus book of Cicero's Academica to denote a space completely filled with matter (distinguished from ināne or vacuum "empty space"); (sense 2) apparently first from its use in Sweden in the eighteenth century to refer to a meeting of all members of the Riksdag — more at full entry 1
First Known Use
1678, in the meaning defined at sense 1a