Non-Aggression Pact
A NAP is IC terminology for Non-Aggression Pact. This indicates a lasting truce between two parties.
They are generally made by family leaders and differ greatly from alliances. They often, and should, contain specific terms such as to what constitutes an attack, what happens in the event of an attack, and how much advance notification should be given before the NAP can be cancelled.
Official NAPs (ones signed via the relations page) have the ability to prevent hostilities (only attacks, special operations are allowed) between the families.
Non-official NAPs do not have this ability, but are more flexible, and often include terms to end them, terms on what to do if someone violates them, and terms the two (or more) leaders express desires on. These are usually made in the form of in-game messages.