Ground Zero

WordReference Random House Unabridged Dictionary of American English © 2026

ground′ ze′ro, 

1. the point on the surface of the earth or water directly below, directly above, or at which an atomic or hydrogen bomb explodes.
2. Informal Terms the very beginning or most elementary level:
   Some of the students are starting from ground zero.
• 1945–50

Collins Concise English Dictionary © HarperCollins Publishers::

ground zero noun

1. a point on the surface of land or water at or directly above or below the centre of a nuclear explosion