Orbifold
From Academic Kids

In topology, an orbifold is a generalization of manifold. It is a topological space (called underlying space) with an orbifold structure (see below). The underlying space locally looks like a quotient of a Euclidean space under the action of a finite group of isometries.
In string theory, the word "orbifold" has a new flavor, and we discuss it in one of the last paragraphs.
The main example of underlying space is a quotient space of a manifold under the action of a finite group of diffeomorphisms, in particular manifold with boundary carries natural orbifold structure, since it is Z_{2}factor of its double. A factor space of a manifold along smooth <math>S^1<math>action without fixed points cares structure of orbifold (this is not a partial case of the main example).
Orbifold structure gives a natural stratification by open manifolds on its underlying space, where one strata corresponds to a set of singular points of the same type.
It should be noted that one topological space can carry many different orbifold structures. For example, consider the orbifold O associated with a factor space of 2sphere along a rotation by <math>\pi^{}_{} <math>, it is homeomorphic to 2sphere, but the natural orbifold structure is different. It is possible to adopt most of the characteristics of manifolds to orbifolds and these characteristics are usually different from correspondent characteristics of underlying space. In the above example, its orbifold fundamental group of O is Z_{2} and its orbifold Euler characteristic is 1.
Formal definition
The formal definition goes along the same lines as a definition of manifold, but instead of taking domains in R^{n} as the target spaces of charts one should take domains of finite quotients of R^{n}.
A (topological) orbifold <math>O<math>, is a Hausdorff topological space <math>X<math> with countable base, called the underlying space, with an orbifold structure, which is defined by orbifold atlas (see below).
An orbifold chart is an open subset <math>U\subset X<math> together with open set <math>V\subset <math>R^{n} and a continuous map <math>\phi:V \to U<math> which satisfy the following property: there is a finite group <math>\Gamma<math> acting by linear transformations on <math>V<math> and a homeomorphism <math>\vartheta:V/\Gamma \to U<math> such that <math>\phi=\vartheta\circ\pi<math>, where <math>\pi^{}_{}<math> denotes the projection <math>V\to V/\Gamma<math>.
A collection of orbifold charts <math>\{\phi_\alpha:V_\alpha\to U_\alpha\}<math> is called orbifold atlas if it satisfy the following properties:
 <math>\cup_\alpha U_\alpha=X<math>,
 if <math>\phi_\alpha(x)=\phi_\beta(y)<math> then there is a neighborhood <math>x\in V_x\subset V_\alpha<math> and <math>y\in V_y\subset V_\beta<math> and a homeomorphism <math>\psi:V_x\to V_y<math> such that <math>\phi_\alpha=\phi_\beta\circ\psi<math>.
The orbifold atlas defines the orbifold structure completely and we regard two orbifold atlases of <math>X<math> to give the same orbifold structure if they can be combined to give a larger orbifold atlas.
One can add differentiability conditions on the gluing map <math>\psi^{}_{}<math> in the above definition and get a definition of differentiable orbifolds on the same way as it was done for manifolds.
Orbifolds in string theory
In string theory, the word "orbifold" has a slightly new meaning. For mathematicians, an orbifold is a generalization of the notion of manifold that allows the presence of the points whose neighborhood is diffeomorphic to a coset of <math>R^n<math>, i.e. <math>R^n / \Gamma<math>. In physics, the notion of an orbifold usually describes an object that can be globally written as a coset <math>M/G<math> where <math>M<math> is a manifold (or a theory), and <math>G<math> is a group of its isometries (or symmetries)  not necessarily all of them. In string theory, these symmetries do not have to have a geometric interpretation.
A quantum field theory defined on an orbifold becomes singular near the fixed points of <math>G<math>. However string theory requires us to add new parts of the closed string Hilbert space  namely the twisted sectors where the fields defined on the closed strings are periodic up to an action from <math>G<math>. Orbifolding is therefore a general procedure of string theory to derive a new string theory from an old string theory in which the elements of <math>G<math> have been identified with the identity. Such a procedure reduces the number of states because the states must be invariant under <math>G<math>, but it also increases the number of states because of the extra twisted sectors. The result is usually a perfectly smooth, new string theory.
Dbranes propagating on the orbifolds are described, at low energies, by gauge theories defined by the quiver diagrams.
History
The Vmanifold of Ichiro Satake (1956) provided the first formal definition of what is now called orbifold. It was renamed this way and popularized by William Thurston (cf. footnote p. 300, William Thurston on 'orbifold' name (http://www.msri.org/publications/books/gt3m/PDF/13.pdf)).es:Orbifold