  The physics of a photonic structure is commonly described in terms of its
apparent geometric dimensionality. On the other hand, with the concept of
synthetic dimension, it is in fact possible to explore physics in a space with
a dimensionality that is higher as compared to the apparent geometrical
dimensionality of the structures. In this review, we discuss the basic concepts
of synthetic dimension in photonics, and highlighting the various approaches
towards demonstrating such synthetic dimension for fundamental physics and
potential applications.
