A plenary session was a decision-making mechanism within the Romulan Empire. During these sessions, votes might be taken on important matters.
In a strategic analysis offered by Augments Jack, Lauren, Patrick, and Sarina during the 2374 negotiations between the Dominion and Federation, it was predicted that if the Federation were to accept the Dominion peace proposal, the Romulans would vote to abandon their non-aggression pact with the Dominion in the next year's plenary session. (DS9: "Statistical Probabilities")