areal

I was familiar with the word "area," of course, but I didn't know 'til recently that the adjective form is "areal."

...you can get the same volume of magnetic dipoles packed into a smaller surface area--hence, better potential areal density for the same signal-to-noise ratio, more or less.

--"Toshiba spins 200-Gbyte, 2.5-in. hard-disk drive," by Ron Wilson, published in EDN, 5 June 2006