Business modelling is the initial and heart activity of BSDM (Business System Development Method). It provides a framework for the businessmen to elicit information of business operations and business rules which are then presented in a high level description - business model.An adequate business model presents a converged view of all managers towards to the business. Therefore, it is an excellent reference for business system design - which includes the design of a software system and manual work flow. Business model can be regarded as one of the initial documents produced by requirement engineers whose analysing domain is business operation. The work present is an attempt to understand and describe concepts embedded in a business model. Primitives used in the model are formalised. Standard practice used in forming these models are captured and described as modelling rules; where each are categorised by the nature of its appliance. A formal language, DefBM (Definition language for Business Model) is under development whose goal is to be able to explicitly present concepts embraced in the informal model, and furthermore to demonstrate the underlying dynamic behaviours of the model.