In thermodynamics, the surroundings or environment is that part of the universe external to the boundary of the system. [1] The regions or parts of the surroundings of interest are those in which an energy interaction takes place with the system across the boundary. Said another way, the surroundings are only of interest if they can have some influence on the evolution of the system. [1]

The original regions of interest in the surroundings to the system or working substance, as defined by French physicist Sadi Carnot in 1824, were the boiler (burning coal), the condenser (a cooling stream of water), and the atmospheric pressure change work transferred through the face of the piston. [2]

References
1. Perrot, Pierre. (1998). A to Z of Thermodynamics, (pg. 294). Oxford: Oxford University Press.
2. Carnot, Sadi. (1824). “Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power.” Paris: Chez Bachelier, Libraire, Quai Des Augustins, No. 55.