Did you even try to [google a bit][1] before asking?

Linear functions are functions of the form $f(x)=a\cdot x+b$ for $a,b \in \mathbb{R}$.

Now try to write $\text{abs}(x)$ in such a form. 

Another way to see it: linear functions are "straight lines" in the coordinate system (excluding vertical lines).


 [1]: http://en.wikipedia.org/wiki/Linear_function