I don't understand what an active s-box is (in reference of design criteria 8 of the DES). 

Because (e.g.) according to DES, all 8 s-boxes are used every round. But this is not was active means.

So my question: What is an active S-Box, and what does it mean when a s-box is active (or when two or three s-boxes are active)?

Thanks for your help.