Digital frequency is the analogue for discrete signals as frequency is to continuous signals. Since a discrete signal is a sequence (merely a series of, typically, numbers) it contains no direct information as to determine the frequency of the corresponding continuous signal.
Just like in frequency, a digital frequency can have values in degrees or radians. However, it is common to represent a digital frequency that has been normalized to either the Nyquist frequency or the sampling frequency. Ergo, it is very important to specify the frequency range.
The following frequency ranges are assuming a signal has been properly sampled by satisfying the Nyquist-Shannon sampling theorem.
Hertz range
The values of a valid signal is on 
cycles per second.
Normalized hertz range
The normalized hertz range is the hertz range divided by the sampling frequency. A valid signal is on 
cycles per sample.
Instead, the normalizing frequency could be the nyquist frequency, which puts a valid signal on 
cycles per sample.
Radian range
Using the conversion from D degrees to Θ radians, 
, the radian range for a valid signal is on 
radians.
Normalized randian range
The normalized radian range is the radian range divided by the sampling rate in radians, which is 
. A valid signal is on radians per sample.
Instead, the normalizing frequency could be the nyquist frequency in radians, which puts a valid signal on cycles per sample.
Clearly a frequency of just "0.1" is insufficient to describe the true frequency of the discrete signal. To remove the ambiguity, it is necessary to specify the range and what normalization frequency was used (if applicable).
See also
