Service composition is a main notion in the Service Oriented Architectures (SOA) paradigm. It provides mechanisms to combine available resources which are exposed as services. These mechanisms try to fulfill a key software engineering challenge: the reusability of existing entities. Typically, a service composition specifies a cooperation between services to reply to some high level goals. Many service composition models were developed. Each of them tries to solve some particular problems. This paper intends to clearly express the relevant notions of these models in a Composite Service MetaModel. This composite approach provides an homogeneous reuse of existent compositions. This metamodel defines all interlaced features and provides a global and explicit vision of the service composition. Moreover, these specifications of interlaced features are the cornerstone toward the definition of auto composition in a composite service: the ability to dynamically modify its architecture and its composition logics according to the environmental context.