What is a client in SAP system?

One SAP system can be used for several independent companies (or subsidiaries of a company). The client is used to separate the data of these companies. Most database tables in an SAP system therefore have the client is a key. This is used e.g. for transactional data, master data and client dependent customizing data. Nevertheless there is also customizing that is valid across all clients (so called cross client customizing.)

In a nutshell: Client is a key field in most database tables to separate data of multiple companies using the same SAP system. Each company has its own client number.