Origin: Latin suffix -ary
Plenary has 3 different meanings across 1 category:
plenary session
"The delegates gathered in the main hall to attend the opening of the plenary session."
full in all respects
"a plenary session of the legislature"
"a diplomat with plenary powers"
Fully attended; for everyone's attendance.
"The plenary session was so well-attended that every member of the council had a seat in the hall."
In plain English: Plenary means complete and total, leaving nothing out or unfinished.
"The plenary session was attended by all members of the committee."
Usage: Do not confuse this with "plenary," which refers to something complete or full in a different sense, though the two are often interchangeable when describing meetings. Use "plenary" specifically to indicate that an event is open to all members of a group rather than just a select committee.
The word plenary comes from the Latin phrase meaning "full," which entered Middle English as a term for completeness. It traveled into modern usage to describe sessions or assemblies where every member is present and voting rights are complete.