In telecommunication

Telecommunication

Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

, the term off-air or off-the-air has the following meanings:

1. In radio
   Radio
   Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
   
    communications
   Telecommunication
   Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
   
    systems, pertaining to a radio station
   Radio station
   Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
   
    or television station
   Television station
   A television station is a business, organisation or other such as an amateur television operator that transmits content over terrestrial television. A television transmission can be by analog television signals or, more recently, by digital television. Broadcast television systems standards are...
   
    that is completely shut down, i.e. that is not transmitting any signal, not even an un-modulated
   Modulation
   In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
   
    carrier wave
   Carrier wave
   In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
   
   . Other terms in this context include "dark" or "silent"
   Dark (broadcasting)
   In the broadcasting industry, dark is a term used to describe a radio station or television station that has gone off-the-air for an indefinite period of time, or as defined by the U.S. Federal Communications Commission , a "silent" station...
   
   , though those terms were typically used for a station that has left the air for an extended period of time, or permanently; while "off-the-air" is generally used for a brief period of time, or for a station that has recently stopped broadcasting.
2. In a radio station, pertaining to a particular source of modulation, such as a specific microphone
   Microphone
   A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
   
   , that is disconnected, i.e. is no longer capable of modulating the carrier.

Note: The carrier wave

Carrier wave

In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...

 may continue unmodulated or it may be modulated by another signal source, such as a subcarrier

Subcarrier

A subcarrier is a separate analog or digital signal carried on a main radio transmission, which carries extra information such as voice or data. More technically, it is an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth...

.

Also, off-the-air may be synonym

Synonym

Synonyms are different words with almost identical or similar meanings. Words that are synonyms are said to be synonymous, and the state of being a synonym is called synonymy. The word comes from Ancient Greek syn and onoma . The words car and automobile are synonyms...

ous with over-the-air

Over-the-air

Over-the-air has several meanings, depending on context. *Generally, over-the-air is synonymous for wireless.*Specifically, over-the-air can have the following meanings or is used in the following contexts:...

 or from-the-air, as in picking up a terrestrial

Terrestrial television

Terrestrial television is a mode of television broadcasting which does not involve satellite transmission or cables — typically using radio waves through transmitting and receiving antennas or television antenna aerials...

 broadcast TV station off-the-air instead of from cable TV. As this usage may be confusing due to the term's other meanings, the terms "over the air" or "on the air" are more standard in the broadcasting industry.