What are Plenary Sessions?

Plenary session is a term often used in conferences to define the part of the conference when all members of all parties are to attend.
These sessions may contain a broad range of content from keynotes to panel discussions and are not necessarily related to a specific style of delivery.

The term has been used in the teaching profession to describe when information is summarized. This often encourages class participation.
from: http://en.wikipedia.org/wiki/Plenary_session