An interstellar agreement on the humane treatment of prisoners of war, accepted by both the Federation and the Cardassian Union, which the Cardassians evade when they capture and torture Captain Picard.
The Seldonis IV Convention sets out how prisoners of war are to be treated, in much the manner of the Geneva Conventions of old Earth. Both the Federation and the Cardassian Union are parties to it.
It is discussed in the Next Generation two-part story Chain of Command. After Picard is captured on Celtris III during a covert raid, Gul Lemec tells Captain Jellico that Picard will receive the convention's protection only if the Federation admits he was acting under orders. Otherwise he will be dealt with as a terrorist.
Picard himself appeals to the convention while he is held by Gul Madred, who dismisses it and continues the interrogation. The dispute shows how easily the agreement could be set aside when a government wished to deny that a mission had ever been authorised.