What is Collective Synchrony?Synchronization is a general nonlinear phenomenon which can be briefly described as an adjustment of rhythms of self-sustained oscillatory systems due to their interaction [1]. In the simplest setup, two periodic oscillators of this class can adjust their phases and frequencies, even if the coupling between the systems is very weak. This effect is often called phase locking or frequency entrainment. The concept can be generalized to the case of many oscillating objects with a variety of different coupling configurations: oscillating subsystems can be placed on a regular lattice, or organized in a complex, possibly irregular, network, or form a continuous medium. If an oscillating unit is interacting with its neighbors only, then synchronization typically appears in a form of waves or oscillatory modes. Contrary to this, if the interaction is a long-range one, then global synchrony, where all or almost all units oscillate in pace, can set in. Examples of synchronous dynamics range from mechanical systems like pendulum clocks [2, 3] and metronomes [4], through modern physical devices, e.g., Josephson junctions and lasers (see [5,6] and references therein) to live systems (see, e.g., a review [7]), including human beings [8,9]. The mostly popular model, describing collective dynamics in a large population of self-sustained oscillators is that of globally (all-to-all) coupled units. This framework has been used to describe many physical, biological, and social phenomena. The main effect observed in these models is the collective synchrony, when a large part or all units adjust their rhythms and produce a nonzero mean field, which has the same frequency as the synchronized majority. In the simplest setup this state appears from the fully asynchronous one via the Kuramoto transition [10,11]: if the coupling strength ε in the ensemble exceeds some threshold value ε cr , the macroscopic mean field appears and its amplitude growth with the super-criticality ε − ε cr . This transition is often considered in an analogy to second order phase transitions; on the other hand, it can be viewed at as a supercritical Hopf bifurcation for the mean field.