Several thermodynamic definitions are very useful in thermochemistry. A system is the specific portion of the universe that is being studied. Everything outside the system is considered the surroundings or environment. A system may be:
* a (completely) isolated system which can exchange neither energy nor matter with the surroundings, such as an insulated bomb calorimeter
* a thermally isolated system which can exchange mechanical work but not heat or matter, such as an insulated closed piston or balloon
* a mechanically isolated system which can exchange heat but not mechanical work or matter, such as an uninsulated bomb calorimeter
* a closed system which can exchange energy but not matter, such as an uninsulated closed piston or balloon
* an open system which it can exchange both matter and energy with the surroundings, such as a pot of boiling water