C/SIDE is the development environment for Microsoft Dynamics NAV / Navision and is short for Client/Server Integrated Development Environment. The actual development language is called C/AL.
In daily use C/SIDE is referred to as the development platform for Navision as integrated into the client software of Navision.
When changing the NAV/Navision application then you are not changing the executable s (the exe files), but the "objects" which are stored in the database containing not only the definition of tables, forms/pages, reports/dataports but also the entire business logic required to run the ERP application.
In Dynamics NAV/Navision you have the different objects: