Definition of dead spot:
(n) :
An area that has no radio (or wi-fi) reception.
(n) :
(cinematography) A part of a scene that has insufficient lighting.
(n) :
(architecture) The flaw of building that cannot be seen when collapsing because poor structure, built not according specified standards, or not earthquake-proof.
(n) :
(seismology) A location around an earthquake epicentre that is prone to earthquake.