A relationship agreement which is common in polyamorous relationships, mainly in primary/secondary relationships. A veto agreement gives a member of the relationship the power and authority to stop the other member from enjoying additional relationships or a particular activity or (sometimes sexual) act.
The power of veto is almost always assigned to the primary member of the relationship.
Becky: “I’m sorry Jason, David is deciding to veto the relationship we have going on…”
Jason: “He can’t do that can he? I thought what we had was special?!”
Becky: “It is special Jason! Well, it was… But, David is the primary.”
Jason: “I understand, that’s it then.”