Entropy (S) is a measure for the part of energy of a system which cannot be used to do work. In a wider sense, it can also be interpreted as a measure for the disorder of a system.
Entropy is defined as
S = k ln Ω
where k is Boltzman's constant and Ω is the number of possible states that a system may be in. This definition of entropy is similar to the definition of entropy in information theory and therefore reveals deep connections between information theory and thermodynamics.