zero point

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun military The location of the center of a burst of a nuclear weapon at the instant of detonation. The zero point may be in the air, or on or beneath the surface of land or water, depending upon the type of burst, and it is thus to be distinguished from ground zero.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the point on a scale from which positive or negative numerical quantities can be measured