quality of being agreeable to reason

Rationality is the state that exists when reason is used. It's also related to moderation in some contexts and sensibility in others.

When a person is reasonable or rational they are using all available information to make a decision, and whether their decision can be judged as rational or reasonable depends on the reasons they made it. If they believe that the most important factor in any decision is how they will personally benefit, then self-interested or even selfish behavior would be rational. But if they believe that benefiting a group of people is more important than benefiting just one person, then purely selfish behavior is irrational. Determinations of rationality depend on context, the background information and assumptions.