What does Photomultiplier tube sensitivity mean? When people say that it is highly sensitive, how do they measure that?
As Nugatory said, you need to be a bit more specific here, because "sensitivity" can mean different things in different context.
The context that *I* am familiar with is what is known as the device quantum efficiency. This measures how responsive the PMT is to incoming photons. If I send out 100 photons, how many of those can trigger a signal in the PMT?
We measure that using a calibrated light source (i.e. we have calibrated the wavelength and power of the impinging beam), and we look at the amplitude of the signal.
This may or may not correspond to what you are looking for, but without any further elaboration, no one knows.