Definition of dead air:
(n) :
(idiomatic, broadcasting) An unintended interruption in a broadcast during which nothing is transmitted: a silent period on radio or a silent period with no picture on television.
(n) :
(idiomatic, by extension) A quiet or awkward moment in a conversation where neither individual is speaking.
(n) :
(mechanical, HVAC, engineering) Air that is subject to little or no ventilation, thus, little to no air exchange.