Modbus protocol is application layer protocol used for serial communication between devices, mainly to exchange data typical for the field of automation.
At this level Modbus is a stateless client-server protocol (e.g. much like HTTP), based on transactions, which consist of a request (issued by the client) and a response (issued by the server). In the field where this protocol is usually applied, there exists a concept that is one of the possible schemas governing the lower level communication behavior on a network using a shared signal cable: Master-Slave. To prevent confusion, the following directed relations describe Master-Slave in terms of the Client-Server paradigm:
the Master is a Client
the Slave is a Server