The rec.arts.tv FAQL:

Last updated:  February 1, 2004.
This version:  8.00

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Section 3.3: "Where did the term 'channel' come from?"

Answer:

[NOTE: This section, edited by me, was constructed from a post by 
       David Chesler (chesler@world.std.com) in response to a question 
       on this topic in rec.arts.tv.]

The bandwidth of an AM radio signal is so narrow that for all practical purposes it's one point on the dial, and a station can be correctly identified as "1340 Hertz". The same is also roughly true of FM radio signals.

But a TV signal takes a big chunk of the spectrum, so the FCC assigned a series of separate chunks for use in TV broadcasting back when TV was first starting. But it would have been awkward to call each chuck as "in between 45 and 55 megacycles on your dial." Besides, the TV sets were just going to have pretuned settings for each chunk of airwave anyway, so they had to call them something. At this early date in the history of TV, the term "channel" was already in use in commercial two-way radio and was used by the FCC to describe FM radio assignments (which are a lot wider, in terms of spectrum, than AM radio signals), although FM stations were still identified to the public as "93.5 on your dial."

The deiniftion of a "channel" is basically a groove of finite width in the spectrum, so it embraces the idea of a wide range of frequencies, as opposed to just one frequency. It makes more sense to refer to frequencies when they are evenly spaced (as they tend to be in radio), but with the odd allocation of TV channels, a simple identifier made more sense. So the term "channel" was adopted into wide use to desribe TV signals.

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