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It is hard to define drought because it means something different in different areas of the world. In a very arid region there has to be a really long period without rain for it to be considered a drought event. However, in tropical areas six days without rain is a period of drought.
The definition of drought, therefore, depends on the place on Earth where we are. Is this definition good enough? Not really! The definition of drought also depends on the demand that people living in the area place on the water resources. This, of course, depends on the number of people living in the region. Think about it. If there is a period of reduced rainfall, but nobody lives in the area, can we really say this is a drought? It is, because rainfall amounts are less than normal but no one has been affected by the reduction. On the other hand, there might be a reduction in rainfall in a very populated area where local agriculture is a very important food source. We would, of course, consider this a drought and the consequences of this would be large both because water is needed for human life and for animals and crops.
Whatever the definition, it is clear that drought cannot be viewed solely as a physical phenomenon since it's impact depends on how much water is needed by society. The presence of humans often exacerbates the impact of drought. Recent droughts have brought home the vulnerability of all societies to this “natural” hazard. How vulnerable a part of the world is to drought depends on it's climate, it's type of agriculture, how developed it is and how it manages it's water resources.
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