# Correct definition of frequency density - histograms

What is the correct definition of frequency density?

According to a textbook I am teaching from, frequency density is merely proportional to $\dfrac{\text{frequency}}{\text{class width}}$, and could be a multiple of $\dfrac{\text{frequency}}{\text{class width}}$.

This seems wrong to me. For example, one would not say that population density could be $2\times\dfrac{\text{population}}{\text{area}}$. So why is it OK to do this with frequency density? Is the textbook misusing the term 'frequency density'?