Although sedation for symptom relief in terminally ill patients has been the focus of recent medical studies, the interpretation of research findings is difficult due to the confusing terminology. To clarify the agreements and inconsistencies in the definitions of sedation, a systematic review was performed. We searched the literature through MEDLINE from 1990 to July 2001. A total of 7 articles met the inclusion criteria. All studies included the use of sedative medications or the intention to reduce patient consciousness as an essential element of sedation. All but one study explicitly described that the primary aim of sedation was symptom palliation. Three definitions stated that target symptoms were severe, and 4 articles reported the refractory nature of the distress. On the other hand, there were marked inconsistencies in the definition of the degree of sedation, duration, pharmacological properties of medications used, target symptoms, and target populations. This review suggests that sedation includes two core factors: the presence of severe suffering refractory to standard palliative management, and the use of sedative medications with the primary aim to relieve distress. Thus, "palliative sedation therapy" can be defined as "the use of sedative medications to relieve intolerable and refractory distress by the reduction in patient consciousness." The marked inconsistencies in the definition of sedation should be considered to be subcategories of palliative sedation therapy, and we recommend that researchers define the degree of sedation, duration, pharmacological properties of medications, target symptoms, and target populations in future studies. This clarification of terminology will contribute to improving the accuracy and depth of sedation research.