Since the attacks on September 11, 2001, the world has struggled to define al-Qaeda, an amorphous, growing, and seemingly inexhaustible organization. Once a relatively organized group based in one country with a defined hierarchy and clear leadership, al-Qaeda has transformed into a transnational phenomenon over the last few decades, with branches and affiliates operating in dozens of countries across the world. Many call al-Qaeda an enemy, while some define it as an ideology, and others analyze it as a network. Of course, a small minority takes it up as their cause and an extension of their religion.

The difficulty of defining al-Qaeda is reflected by the various ways the Arabic word qaeda can be translated. The most common translation is "base" or "foundation", but the word can also be used more ambiguously to mean "method", "principle", or "formula". Some maintain that the early founders of al-Qaeda were envisioning the name in the latter sense, while others believe that the name was applied to the group by outside powers. Osama bin Laden, the founder of al-Qaeda, stated in a February 2002 interview that the name al-Qaeda was "established a long time ago by mere chance", and that the group's training camp was called al-Qaeda ("the base"), a name that came to represent his entire organization.

Even greater frustration arises when attempting to physically identify what group or groups constitute al-Qaeda. Are all groups that pledge allegiance to al-Qaeda considered part of the franchise? Is there a difference between the "al-Qaeda core" based in Pakistan and the various affiliates and associates scattered across the world. If so, what is the relationship between them? While both groups have sworn allegiance to al-Qaeda's core leadership, is al-Qaida in the Arabian Peninsula in Yemen fighting for the same cause as al-Shabaab in Somalia or ISIS in Iraq and Syria?