where δ Q {\displaystyle \delta Q} is the infinitesimal amount of heat absorbed by the system from the reservoir and T surr {\displaystyle T_{\text{surr}}} is the temperature of the external reservoir (surroundings) at a particular instant in time. The closed integral is carried out along a thermodynamic process path from the initial/final state to the same initial/final state. In principle, the closed integral can start and end at an arbitrary point along the path.