Definition of cluster according to M.E. Porter
Cluster- a geographic concentration of interrelated companies, specialized suppliers, service providers, companies which operate in the same field and associated institutions (for example universities, supervisory units, trade associations and financial institutions) in their respective fields, competing but also cooperating. Clusters reaching critical mass (the necessary number of companies and other institutions making up the agglomeration effect) and achieving competitive success in the fields of their interest, are a remarkable feature of almost every national economy, regional, state, even the metropolitan, mostly in economically developed countries.

Definition of an innovative cluster by European Commission
Innovative clusters - concentration of independent businesses - newly founded innovative companies, small, medium and large enterprises and research organizations - operating in a particular sector, region and founded to stimulate innovative activity by promoting intensive connections, sharing background facilities and exchange of knowledge and expertise and by contributing effectively to technology transfer, networking and sharing of information among companies within the cluster. Member countries should aim to achieve a proper balance between SMEs and large companies in the cluster to achieve certain critical mass, especially by specialization in a certain area of R & D & I and taking into account the existing clusters in the Member States and at Community level.