  Dissociation of quarkonium in quark-gluon plasma (QGP) is a long standing
topic in relativistic heavy-ion collisions because it has been believed to
signal one of the fundamental natures of the QGP -- Debye screening due to the
liberation of color degrees of freedom. Among recent new theoretical
developments is the application of open quantum system framework to quarkonium
in the QGP. Open system approach enables us to describe how dynamical as well
as static properties of QGP influences the time evolution of quarkonium in a
coherent way.
  Currently, there are several master equations for quarkonium corresponding to
various scale assumptions, each derived in different theoretical frameworks. In
this review, all of the existing master equations are systematically rederived
as Lindblad equations in a unified framework. Also, as one of the most relevant
descriptions in relativistic heavy-ion collisions, quantum Brownian motion of
heavy quark pair in the QGP is studied in detail. The quantum Brownian motion
is parametrized by a few fundamental quantities of QGP such as real and
imaginary parts of heavy quark potential (complex potential), heavy quark
momentum diffusion constant, and thermal dipole self-energy constant, which
constitute in-medium self-energy of a static quarkonium. This indicates that
the yields of quarkonia such as $J/\psi$ and $\Upsilon$ in the relativistic
heavy-ion collisions have the potential to determine these fundamental
quantities.
