There is no single, quantifiable definition of drought, but a good, practical definition of drought is that it occurs when precipitation and other sources of water add up to less than we expect or need.
Inadequate rainfall can cause drought, but so can increasing demands on water supply.
There are both physical and social factors that add up to drought. Lack of rain is a physical event that we can't really change. Our habits and expectations surrounding water use are the social side of drought. Drought's impacts are the cumulative result of both physical and social factors. We can reduce vulnerability to drought by doing a better job of balancing water supply and demand.