Comfort is a term that has a significant historical and contemporary association with nursing. Since the time of Nightingale, it is cited as designating a desirable outcome of nursing care. Comfort is found in nursing science, for example in diagnostic taxonomies, and in references to the art of nursing, as when practice is described. Roy, Orlando, Watson, Paterson and others use comfort in major nursing theories. The term can signify both physical and mental phenomena and it can be used as a verb and a noun. However, because comfort has many different meanings, the reader has had the burden of deciding if the term is meant in one of its ordinary language senses or if its context reveals some special nursing sense. The purpose of this paper is to analyse the semantics and extension of the term 'comfort' in order to clarify its use in nursing practice, theory and research. The semantic analysis begins with ordinary language because the common meanings of the term are the primary ones used in nursing practice and are the origin of technical nursing usages. Comfort is discussed as the term is found in nursing, including texts, standards of care, diagnoses and theory. An account of patient needs assessment is used to cull three technical senses of the term from its ordinary language meanings. After contrasting these senses in order to justify their separateness, they are shown to reflect differing aspects of therapeutic contexts. Defining attributes of the three senses are then explicated and presented in table format. The last section of the paper addresses some of the ways that the extensions of the senses can be measured.