The DIESIS project  gives the following definition:
"A conceptual schema or conceptual data model is a map of concepts and their relationships. This describes the semantics of an organization and represents a series of assertions about its nature. Specifically, it describes the things of significance to an organization (entity classes), about which it is inclined to collect information, and characteristics of (attributes) and associations between pairs of those things of significance (relationships)." 
Other International Definitions
Model in its broadest sense is a representation of how a system works and can be used to understand how the system will respond to inputs and other changes.