Young massive star clusters
  Young massive clusters are dense aggregates of young stars that form the
fundamental building blocks of galaxies. Several examples exist in the Milky
Way Galaxy and the Local Group, but they are particularly abundant in starburst
and interacting galaxies. The few young massive clusters that are close enough
to resolve are of prime interest for studying the stellar mass function and the
ecological interplay between stellar evolution and stellar dynamics. The
distant unresolved clusters may be effectively used to study the star-cluster
mass function, and they provide excellent constraints on the formation
mechanisms of young cluster populations. Young massive clusters are expected to
be the nurseries for many unusual objects, including a wide range of exotic
stars and binaries. So far only a few such objects have been found in young
massive clusters, although their older cousins, the globular clusters, are
unusually rich in stellar exotica. In this review we focus on star clusters
younger than $\sim100$ Myr, more than a few current crossing times old, and
more massive than $\sim10^4$ \Msun, irrespective of cluster size or
environment. We describe the global properties of the currently known young
massive star clusters in the Local Group and beyond, and discuss the state of
the art in observations and dynamical modeling of these systems. In order to
make this review readable by observers, theorists, and computational
astrophysicists, we also review the cross-disciplinary terminology.
