A Mutual Defense Pact, or MDP, is a type of alliance treaty wherein the signatory alliances agree to defend each other from an attack by other alliances.
A Mutual Non-Chaining Defense Pact, or MnDP, is a variant of the traditional MDP. MnDPs have a non-chaining clause included. This means that if one signatory assists another alliance via a separate treaty, the treaty with the non-chaining clause becomes optional. These are used to limit the expansion of conflicts if an alliance doesn't want to be involved for one reason or another.