(1) A dry period, especially a long one. Usually defined as creating a deficiency of soil moisture in the "plant root zone".
(2) A dry period long enough to deplete soil moisture enough to seriously retard plant growth.
(1) I'm a plant. I detected deficient soil moisture in my root zone. "Drought," I concluded. Then someone poured beer on my stalk and everything got weird.
(2) Everything was parched. It was a severe drought. All the Eds wilted.