The aim of this article is to define and operationalize the construct of infantilism. The methods of theoretical research involve analysis and synthesis. Age and content criteria are analyzed for childhood and adulthood. Infantile traits in an adult are described. Results: The characteristics of adult infantilism in the modern world are defined, taking into account increasing information flows and socio-economic changes. The concept of "infantilism" and its main features are defined as an organization of the personality that includes traits and behavioral models that are typical of one's earlier age periods and not appropriate to the person's actual age, which is most articulately manifested in emotional and volitional immaturity of an individual. Scientific novelty: The main psychological characteristics of adulthood are described, including reflection, the desire to work and have a vocation, vocational self-determination, work skills, the desire for self-realization, and emotional and volitional maturity. Objective characteristics of adulthood are: transition to economic and territorial independence of the parental family, and development of new social roles, such as that of worker, spouse, and parent. Two possible operationalizations of the concept are identified: objective (existence/absence in a person's life of objective criteria of adulthood) and subjective (self-report on the subjective feeling of existence/absence of the psychological characteristics of adulthood). The practical significance of the work consists in formulation of an operationalization of "infantilism," which at the moment has so many interpretations. Such operationalization is necessary for further analysis and research. [This article was translated by Susan Welsh.]