The Word 'disaster' is believed to be originating from the French word 'Disastre' which is a combination of two words 'des' meaning bad and 'astre' meaning star. Thus this term refers to a 'bad star'. It may be defined as the effect of a natural or manmade hazard that causes an enormous loss to human life together with plants and animals. According to the united nation, "disaster is a serious disruption of the functioning of a community or a society causing wide spread human material, economic and environmental losses which exceed the ability of the affected community or society to cope with using its own resources".

       A disaster is a result of combination of hazard, vulnerability and insufficient capacity to reduce the chances of damage. A disaster occurs when a hazards strikes on the vulnerable areas and causes damage, casualties and disruption. Without vulnerability or hazard there is no disaster. It occurs when both hazard and vulnerability meet.

Any hazards like flood, earthquake or cyclone together with greater vulnerability (like inadequate supply of resources, sick and old people, lack of awareness etc.)  would lead to disaster causing greater loss to life and property. For example, an earthquake of strong intensity can be disaster only when it affects people, property and their activities on large scale not when it takes place in some deserted place.

                     Disasters occur rapidly and spontaneously. It is an extreme event which surpasses the tolerable limits causing great loss of life and property. Hazardous extreme environmental process cannot be termed as disasters until they affect the humans adversely. For example, a very strong tropical cyclone is an extreme event when it occurs and dies in the midst of an ocean but it becomes a disaster when it strikes the inhabited coastal areas. Similarly, a volcanic eruption is an extreme event but becomes a disaster when it occurs in densely populated area.

The consequences of environmental disasters are devastating for the social, economical and environmental system of a region or for the global ecosystem. They do not recognize the man made borders and are threat for future generations as it can abide them from a clean and friendly environment. Environmental disasters threaten life for long period of time. Their short term effects include injuries, poisoning. Radiation etc. but long term effects do not happen immediately and take some time to come to the surface for example malignant tumours and genetic defects which occur from the accumulation of hazardous materials.