# What does Integral of $d^2$ mean

I was going through the Gravitation Lensing Wikipedia page and came across this formula:

This notation of the Integral doesn't say anything to me.

Why is the expression after the $d$?

What does $d^2$ in this context mean?

• This is common notation in physics to denote an area integral. Similarly $d^3 x$ or something similar denotes a volume integral. – Alex Jun 22 '17 at 21:18

$d^2\theta '$ means that you're integrating in two dimensions (it's a double integral, on both directions of $\theta'$.
$d^2 \theta^{'} = d\theta_x d\theta_y$