Heterogeneity of schema and data in information integration complicates metadata management and query processing. We consider a mediator-based approach (MI) to information integration and propose a model theoretic approach to describe integration. In this model, we view the schema of the integration as a 3D space, and assume it is a complete lattice. The first dimension in the model specifies the concepts (e.g., entity sets, relations, classes, etc), the second dimension indicates the data model in which a concept is represented (e.g., relational, semi-structured, object-oriented, etc), and the third dimension gives the application domain. We also introduce three basic transformations, called X-transform, Y-transform, and Z-transform, to all of which we refer as primitive queries and show how user queries can be expressed using primitive queries. We show a typical architecture and illustrate usefulness of this model as it generalizes the mediator based information integration in which the global schema is basically the least upper bound of all the points in the 3D space.