A surface/object with only one side, and therefore Euler characteristic of 0.
Technically two sides as a one sided object is an infinitely small point.
Technically nothing that exists within our world can be anything short of a 3D shape with multiple sides. Any "box" you own for instance will "technically" have thousands on thousands of sides.
You don't take that into account when looking at the theory though, and conceptually a mobius strip really is one sided![]()
Fair enough , after all what do I know I am a part time gynaecologist![]()
Sort of started as a hobby and grew from there*shrugs* better job than me
Crack up every time I think of the Red Dwarf gynaecologist cake gag.