Why is Rf called Retardation Factor? I understand that $$R_\mathrm{f} = \frac{\text{distance traveled by center of analyte spot } (b)}{\text{ distance travelled by solvent front } (a)}$$ What I do not understand is why this is called retardation factor; an analyte that travels further in the mobile phase seems to me to be less retarded by the stationary phase, while one that doesn't travel as far seems more retarded. If we're using this terminology why doesn't $R_\mathrm{f} = 1- \frac ba$?
The clearest answer is that an analyte is retained by both the mobile and the stationary phase and the retention factor is a measure of the ratio between retention in the mobile phase and in the stationary phase. 
So you could think of it as a higher Rf value indicates a higher retention in the mobile phase.

(Also, in answer to some of the comments, the retardation factor is the inverse of the retention factor)