Möbius strip

Short definition from Wiktionary

Noun

1. 1
   A one-sided surface formed by identifying two opposite edges of a square in opposite senses;
2. 2
   A one-sided surface formed by identifying two opposite edges of a square in opposite senses; a narrow strip given a half twist and joined at the ends, forming a three-dimensional embedding of said surface: a narrow strip given a half twist and joined at the ends, forming a three-dimensional embedding of said surface.