实际上就是相对于有线电视（cable TV）而言的无线电视，所谓无线和有线主要是从信号接收的角度来说的。

介词 off 可以表示两种几乎相反的意义：一个是“分离”，这个最常见。还有就是表示“依靠”，如 live off one's pensions 靠养老金生活，This little gadget works off battery. 这个小玩意儿靠电池来工作。

相应地，off-air 用在通信领域也有截然不同的意思，上海译文《英汉大词典》：off-air adj. & adv. 1. 直接从无线电广播；2. 通过电缆广播

http://myhard.yesky.com/455/2036455.shtml
ATI全新Theater方案进军数字电视市场
2005-07-15 23:13
在这样的情况下，导致价位高但可能拥有较好讯号接收的大尺吋数字电视，却被连接至有线电视或卫星视频转换器。而内建较低价接收器的中低阶的数字电视，却被应用在无线(off-air)接收，一旦模拟电视频号停止播送时，大大提高无法收视的风险。通过ATI的Theater 311、Theater 312以及Theater 314接收芯片解决方案，将能协助业界在进行数字化的过程中避免上述窘境。

http://www.compotech.com.cn/articleinfo.php?cid=27&id=1398
数字电视的发展趋势:从美规产品兼容性谈起
数字电视机内建ATSC调谐器，从硬件结构来说，是一个单纯的产品化问题，然而，真正的困难不在此，厂商们碰到的第一个议题是: 要支持地面广播就好了，还是要将数字有线电视包括在内? 没错，以地面接收方式(off-air)来看电视的美国家庭估计不到10%，当大多数人以有线电视或卫星电视为主要节目来源时，显然只有ATSC地面广播是不够的。

http://cn.tradekey.com/profile_view/uid/1460342.htm
Thankyou for choosing Southern Prewire a company for all your cable and off air solutuions. with over 20 years experience we can expertly solve all inhome cable problems, which your cable provider cannot, and all problems and concerns dealing with goverment digital transition.

http://www.answers.com/topic/off-the-air

Wikipedia: off-the-air
In telecommunication, the term off-the-air has the following meanings:

In radio communications systems, pertaining to a station that is completely shut down, i.e., that is not transmitting any signal, not even an un-modulated carrier wave. Other terms in this context include "dark" or "silent", though those terms were typically used for a station that has left the air for an extended period of time, or permanenently; while "off-the-air" is generally used for a brief period of time, or for a station that has recently stopped broadcasting.
In a radio station, pertaining to a particular source of modulation, such as a specific microphone, that is disconnected, i.e., is no longer capable of modulating the carrier.
Note: The carrier may continue unmodulated or it may be modulated by another signal source.

Also, off-the-air may be synonymous with over-the-air or from-the-air, as in picking up a terrestrial 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.