Extent of randomness in a system or with the extent to which energy is distributed among the various motions of the molecules of the system.

S_final - S_initial

For an isothermal process:
ΔS = q_rev/T

ΔS for phase changes:
ΔS = ΔH_fusion/vap/T

Reversible Process:
ΔS_univ = ΔS_surr + ΔS_sys = 0

Irreversible Process:
ΔS_univ = ΔS_surr + ΔS_sys > 0

Properties that define a state and do not depend on how we reach that state.