temperature of a system is constant

According to the first law of thermodynamics "energy can be changed from  one to another but it can neither be created nor destroyed. The energy in our surroundings remains constant which merely changes. 

There are three type of systems in thermodynamics that are following:

1. Open system
2. Closed system
3. Isolated system

• The system across the boundary of which transfer of both mass as well as energy can take place across the boundary is called as open system. Example: Turbine
• The system of fixed mass across the boundary of which no mass transfer can take place is called as closed system. Example: Cooking pressure cooker
• The system in which both the mass as well as energy content remains constant is called an isolated system. Example: Thermal flask