Abstract: As a hotspot of data-warehouse, there are several methods to define metadata,such as data about data,a bridge between data and information,these definitions were put forward, depending on different applications,and they can not express the significance of metadata. The paper began from studying the process that metadata was departed from underlying data,then the classification and definition of metadata were studied, at last we came up with the metadata in general sense and the metadata in narrow sense, these definitions overcome the shortcomings of the definition oriented special applications.