  Quasi-elliptic cohomology is a variant of elliptic cohomology theories. It is
the orbifold K-theory of a space of constant loops. For global quotient
orbifolds, it can be expressed in terms of equivariant K-theories. Thus, the
constructions on it can be made in a neat way. This theory reflects the
geometric nature of the Tate curve. In this paper we provide a systematic
introduction of its construction and definition.
