There continues to be considerable confusion and controversy on the use of diagnostic and clinical terms assigned to patients with severe alterations in consciousness. This confusion results largely from the lack of a uniform classification system that is based on behaviorally defined criteria. This position paper provides recommendations for defining coma, vegetative state (including persistent and permanent vegetative state), akinetic mutism, the minimally responsive state, and locked-in syndrome based on neurobehavioral and neuropathologic features. Current controversies surrounding use of these terms also are discussed.