This concept analysis examines how the concept of intimacy has been defined in the scholarly literature in order to determine what characteristics or necessary conditions exist that differentiate this phenomenon from all other phenomena. The objective of this concept analysis is to derive a theoretical definition of intimacy that can then be used to operationalize the concept for research. Intimacy is of significance to psychiatric nursing theory because (a) intimacy has been identified as being important to the psychosocial development of adults; (b) intimacy plays a developmental role in identity formation, through the consensual validation of personal worth by providing individuals with the opportunity to feel understood and accepted as they are, within the relationship; (c) research indicates that married couples reporting a deficiency or an absence of intimacy have a significantly higher proportion of symptoms of a nonpsychotic emotional illness; (d) failure to develop intimacy in relationships has been identified as one of the most common factors for seeking outpatient psychotherapy; and (e) intimacy has been identified as a key component in individuals' satisfaction with their social support. The theoretical definition for intimacy is this: a quality of a relationship in which the individuals must have reciprocal feelings of trust and emotional closeness toward each other and are able to openly communicate thoughts and feelings with each other. The conditions that must be met for intimacy to occur include reciprocity of trust, emotional closeness, and self-disclosure. As the literature does not uniformly incorporate physical intimacy as a necessary condition for the concept of intimacy, physical closeness was not incorporated into the definition.