I'd like to ask a question on "Asymmetry of Outer Space" by Yael Algom-Kfir & Mladen Bestvina.

In Example $2$, page $4$ it says "Note that in this case the asymmetry can be explained by the fact that the injectivity radius $\text{injrad}(x_k)$ of $x_k$ goes to $0$, and in fact $d(x_k,x_2)\sim -\log{\text{injrad}(x_k)}$".

My problem is that I don't know what injectivity radius is. Neither in the specific case of $CV_n$, nor generally.

Could someone explain or provide some relevant material to study?

Thanks in advance!