ground zero

  1. the land or water surface area directly below or above the point of detonation of a nuclear bomb
  2. Slang
    1. nothing
    2. the starting point
    3. the most basic condition or level

See ground zero in American Heritage Dictionary 4

noun
  1. The target of a projectile, such as a missile or bomb.
  2. The site directly below, directly above, or at the point of detonation of a nuclear weapon.
  3. The center of rapid or intense development or change: “The neighborhood scarcely existed five years ago, but today it is the ground zero from which designer shops and restaurants radiate” (Robert Clark).
  4. The starting point or most basic level: My client didn't like my preliminary designs, so I returned to ground zero.
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