A PPLIED sociology consists today more largely in the application of serviceable concepts and in tested techniques of social research, than in the application of laws or principles, which are few in verified number and usually too general in statement to solve some concrete problem. Consequently it is to the serviceable sociological concept that we turn for some of the examples of fruitful applications of sociology. Social work and psychiatry are making increasing use of sociological concepts. This development is too apparent to be elaborated or even illustrated at this point. Disagreement among definitions of the same concept are frequent as Eubank' has shown, but, if the resulting arguments and discussions are not too protracted, they are probably indications of a healthy condition of normaal growth of scientific vocabulary. We come, therefore, to the question: What is a definition? It becomes necessary to try to define what we mean by the term definition as used in the description and differentiation of sociological concepts. In this brief paper we shall set down some criteria and distinctions that we have found helpful in our own thought, research, and writing. If we begin with dictionary definitions, we find in Oxford that a definition is, "stating the precise nature of a thing or the meaning of a word." Standard states that a definition is, "the act or product of marking out, or delimiting, the outlines or characteristics of any conception or thing; determining the elements, attributes, or relations of one object so as to distinguish it, whether as an individual or one of a class, from other objects." Concepts are general ideas and involve abstract thinking. Young2 states the matter thus: "Abstract thinking constitutes the highest form of anticipatory response," and again, "Reasoning is a form of anticipatory response in which the behavior is foreseen in terms of images, ideas or concepts before it actually passes over into consummatory or overt conduct." Sapir3 says, ". . . the speech element 'house' is a symbol, first and foremost, not of a single perception, or even of the notion of a particular object, but of a 'concept,' in other words, of a convenient capsule of thought that embraces thousands of distinct experiences and that is ready to take on thousands more." In Warren's Dictionary of Psychology, we find a concept defined as, "A