The most common tool to help define a process is called a SIPOC diagram. The SIPOC helps team members determine process activities, stakeholders, and boundaries. A SIPOC shows the Suppliers, Inputs, Process, Outputs, and Customers of a process.
The image below shows the suppliers and the inputs into a process in terms of what they add to the process; then subsequently it shows the outputs of the process and its customers. This type of document gives an organization a high-level view of the process and supplier/customer relationships. Notice the column headings.