where S {\displaystyle S} is the thermodynamic entropy of a particular macrostate (defined by thermodynamic parameters such as temperature, volume, energy, etc.), W is the number of microstates (various combinations of particles in various energy states) that can yield the given macrostate, and kB is Boltzmann's constant. It is assumed that each microstate is equally likely, so that the probability of a given microstate is pi = 1/W. When these probabilities are substituted into the above expression for the Gibbs entropy (or equivalently kB times the Shannon entropy), Boltzmann's equation results. In information theoretic terms, the information entropy of a system is the amount of "missing" information needed to determine a microstate, given the macrostate.