What is a Plenary Session?
A Plenary Session (or plenary) is the part of the meeting or conference that everyone attends. The term is used most often in meetings that also include time spent working in smaller groups.
For example, at a conference, the opening keynote may be considered a plenary session, since everyone is expected to attend as that's the only thing on the conference schedule at that time. During a smaller meeting or workshop, the leader may ask everyone to re-group "in plenary" following time spent working in pairs, in small breakout groups, and after breaks.