What is the aspect ratio of a wing?
The Aspect Ratio ($AR$) is defined as the ratio (hence the name) between the wingspan squared $b^2$ and the area of the wing $S$.

Since both $b^2$ and $S$ are measured with a length squared (e.g., $m^2$), $AR$ is an dimensionless quantity.

To answer the follow-on question of TomMcW in the comments, $b$ is the distance between the wingtips, no matter the shape of the wing.