How can a signal have no DC component I am reading a nice book on DSP 


  The Scientist and Engineer's Guide to Digital Signal Processing


It mentions 


  In electronics, the mean is commonly called the DC (direct current) value.


How can a signal have no DC component if the mean is the DC component? Surely every signal's values can be averaged?
A signal having a mean-value or DC component of zero is commonly referred to as mean-free or as having no DC component. It does not mean that it cannot be averaged, just that the average comes out as zero.

Might be a little inexact but it is very common.