  Besides entity-centric knowledge, usually organized as Knowledge Graph (KG),
events are also an essential kind of knowledge in the world, which trigger the
spring up of event-centric knowledge representation form like Event KG (EKG).
It plays an increasingly important role in many downstream applications, such
as search, question-answering, recommendation, financial quantitative
investments, and text generation. This paper provides a comprehensive survey of
EKG from history, ontology, instance, and application views. Specifically, to
characterize EKG thoroughly, we focus on its history, definition, schema
induction, acquisition, related representative graphs/systems, and
applications. The development processes and trends are studied therein. We
further summarize prospective directions to facilitate future research on EKG.
