It is not possible for persons within and between the many disciplines involved in disasters to communicate with each other without clear definitions of the specific terms that are used. In many instances, the same terms have different meanings in different disciplines. Thus, a standardized set of definitions is provided in this chapter and in the associated Glossary of Terms. Some definitions discussed in this Chapter include: (1) disaster; (2) medical disaster; (3) hazard; (4) risk; (5) prevention; (6) modification; (7) event (including onset, duration, amplitude, intensity, scale, and magnitude); (8) impact; (9) mitigation; (10) preparedness; (11) damage; (12) vulnerability; (13) resilience; (14) absorbing capacity; (15) buffering capacity; (16) disaster management; (17) response; and (18) recovery. An annotated, comprehensive set of definitions is provided in the Glossary of Terms in these Guidelines.