What does area density of mm$^{-2}$ mean

Question

I'm not being smart right now but what does an area density of this mean $$10000 \text{ mm}^{-2}$$

Answer 1

Recall that a base to a negative exponent is equivalent to 1/the base to that positive exponent. $(x^{-2} = \frac{1}{x^2})$. Acceleration due to gravity is sometimes listed as 9.8m$s^{-2}$ which is 9.8m/$s^2$ or 9.8m/s/s - 9.8 meters per second per second or "per second squared". Area density is usually mass over area though so a unit is probably needed for the 10000.