Collective defense is an arrangement, usually formalized by a treaty and an organization, among participant states that commit support in defense of a member state if it is attacked by another state outside the organization.
NATO is the best known collective defense organization. Its now famous Article V calls on (but not fully commits) member states to assist another member under attack. This article was invoked after the September 11 attacks on the United States, after which other NATO members provided assistance to the US War on Terror in Afghanistan.
Collective defense has its roots in multiparty alliances, and entails benefits as well as risks. On the one hand, by combining and pooling resources, it can reduce any single state's cost of providing fully for its security. Smaller members of NATO, for example, have leeway to invest a greater proportion of their budget on non-military priorities, such as education or health, since they can count on other members to come to their defense, if needed.
On the other hand, collective defense also involves risky commitments. Member states can become embroiled in costly wars in which neither the direct victim nor the aggressor benefit. In the First World War, countries in the collective defense arrangement known as the Triple Entente (France, Britain, Russia) got pulled into war quickly when Germany declared war on Russia.
Although they overlap, collective defense is different from collective security, in which members work to deter aggression in the first place. The United Nations is an example of a collective security arrangement, albeit with significant limitations. In general, collective defense involves a more explicit commitment among a group of like-minded members.
Another examples of collective defense is West African organisation ECOMOG and Collective Security Treaty Organization (CSTO).