In differential geometry there is the definition of "orientation" of a surface. It means - more or less - that that surface has two faces . By my memories, I would say that - for example -
"the Moebius strip in a non-orientated surface"
Do you know what a Mobius strip is? Take a long rectangle of paper, give it a twist (just once) and then glue together the short opposite edges: now a ladybird can walk over it covering the whole surface , since there is no longer a side "up" and a side ""down". I like it very much (but I prefere the Klein bottle, harder to describe...)
Ciao
Emanuela