What does 'channel' mean? I see many plots like the following that graph counts per channel,

I know what a 'count' is, but I don't know what a 'channel' is. Could somebody please explain to me? My guess is that it is that each channel represents a bin in the histogram, and so channel 453 would be the 453$^{\textrm{rd}}$ bin in the histogram starting from the first bin which would most likely correspond with the, in the case of energy measurements, lowest energy accepted into the histogram.
[Sorry for my ignorance]
 A: In this case a "channel" is a separate register in the data acquisition (assuming the data comes from a MCA or other ADC driven collection device). 
Each channel represents a discrete range in some input to the data acquisition system (generally time or charge) which is linked through the physics of the detector to some physics quantity of interest.
In this case the physics quantity appears to be the energy of a gamma ray (which has presumably been converted to light in a scintillator and the light converted to charge in a PMT or similar detector element).

Or to talk only in terms of your graphs, each of the data points on the graph represents the number of counts that fell into a particle energy range. Those ranges are called channels.

It is, perhaps, worth emphasizing that the word channel gets applied in many different contexts and has very different meanings there. In a context like "t-channel scattering" they refer to ways of drawing the fundamental Feynman-diagrams that can give the same observables. In the context of particle physics data reduction they might refer to different observables that can indicate the same underlying processes or to different processes which can generate the same observable. And I know the word is widely used in other sub-fields as well, but I won't expose my ignorance by trying to give any examples.
