This article explores the notion of business clusters as defined by American economist Michael E. Porter. The phenomenon of clustering is gaining popularity. However, the concept is often misused, which results from an imprecise interpretation of Porter’s definition of the cluster. On the basis of an in-depth critical literature review, several definitions of the cluster used by researchers are discussed in reference to Porter’s definition. Moreover, the main tenets of the cluster concept are presented with a focus on its flexibility. The multitude of cluster definitions and identification methods allows for a wide range of applications. However, it hinders standardization. The article investigates the excessive and inconsistent use of the cluster concept resulting from the flexibility of Porter’s definition. It also aims to identify the most important features of clusters as defined by Porter.