ΔS

In equations, the function:

$\Delta S \,$

where Delta, symbol "Δ", means "large change" (as contrasted with "differential change", symbol "d"), as in before minus after of some quantity, and S is entropy of the system, whereby the product or joint symbol ΔS is the entropy change of the system or reaction on going from an initial state entropy to a final state entropy or in expanded form:

$\Delta S = S_f - S_i \,$