A variation of a content type that stores content outside of SharePoint. External Content Types are part of the configuration for Business Connectivity Services (BCS).
The external content type is a core concept of Business Connectivity Services (BCS). Used throughout the functionality and services offered by BCS, external content types are reusable metadata descriptions of connectivity information and data definitions plus the behaviors you want to apply to a certain category of external data.
External content types enable you to manage and reuse the metadata and behaviors of a business entity, such as a customer or order from a central location, and enable users to interact with that external data and processes in a more meaningful way.