A naturally occurring phenomenon that exists when precipitation has been significantly below normal recorded levels, causing serious hydrological imbalances that adversely affect land resource production systems (Source: http://www.fao.org/ag/againfo/programmes/en/lead/alive_toolkit/pages/pageB_drought_hazard_def.html)

A period of abnormally dry weather sufficiently prolonged so that the lack of water causes a serious hydrologic imbalance (such as crop damage, water supply shortage) in the affected area. (Source: GEMET/MGH)