  The ground state of an homogeneous electron gas is a paradigmatic state that
has been used to model and predict the electronic structure of matter at
equilibrium for nearly a century. For half a century, it has been successfully
used to predict ground states of quantum systems via the local density
approximation (LDA) of density functional theory (DFT); and systematic
improvements in the form of generalized gradient approximations and evolution
thereon. Here, we introduce the LDA for \emph{excited} states by considering a
particular class of non-thermal ensemble states of the homogeneous electron
gas. These states find sound foundation and application in ensemble-DFT -- a
generalization of DFT that can deal with ground and excited states on equal
footing. The ensemble-LDA is shown to successfully predict difficult low-lying
excitations in atoms and molecules for which approximations based on local spin
density approximation (LSDA) and time-dependent-LDA fail.
