Mutual Defense Pacts (often referred to as MDP's) are a type of interalliance treaty wherein the signatory alliances agree to defend each other from attack by other alliances. There is also a variant of the MDP and you may see it sometimes as MnDP. MnDP is the acronym for Mutual Defense Pacts that have a non-chaining clause in them, (here's an example of a non-chaining clause). The non-chaining clause is put into some MDoAP or MADP's so 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 to expansion of conflicts if people don't want to be involved for one reason or another.