"Dead Air is the unintended interruption in a broadcast during which there is no sound."I'd caveat that a bit.
1. It's any noticeable break in programming. Long pauses between words of 2 seconds or so are not dead air, just crappy technique.
2. Dead air is dead air whether intended or not.
3. Dead air is not a synonym for being off air. If you're off air, the vacant space may become occupied by static or a distant station or both. Dead air is silence on the carrier wave. The listener has no way to know if it's on purpose or not.
In a curious March, 2008, ruling, the F.C.C. agreed that WMCU/WKCP did not violate the F.C.C.’s station identification rules when in broadcast a dead carrier, with no station identification, for a three-week period when the station was silent under Commission authority per 47 C.F.R. § 73.1740, The Minimum Operating Schedule:
“In Opposition, Trinity notes that, during the period that the Station was off-air, ‘there were no program offerings to break in between, and the broadcast operation had ended and not yet begun again.’ It asserts that nothing under our Rules requires the hourly identification of an otherwise silent station. While it concedes that the Station’s transmitter carrier continued to operate while the broadcast station was silent, Trinity maintains that ‘there are no station identification requirements for subcarrier uses, and the subcarrier use was provided in accordance with the Commission’s Rules.’”Before the 1920s the term "dead air" just meant stagnant air. In radio it was used to describe the air trapped between layers of sound-proof glass. that's probably how the term was imported into the studio. The first use I come across with the modern definition is in the The Wireless Age Magazine in 1924. But like the OED references the first written use appears most often after the term is formally adopted.