Disagreement is when two people have different beliefs.
Related: Double-Crux, Conversation
Considered of particular relevance to disagreement between people trying to be rational, Aumann's agreement theorem can be informally interpreted as suggesting that if two people are honest seekers of truth, and both believe each other to be honest, then they should update on each other's opinions and quickly reach agreement. The very fact that a person believes something is Rational evidence that that something is true, and so this fact should be taken into account when forming your belief....