Summary.Investigations into the nature of children's definitions commonly imply some classification based on the formal characteristics of the definition (definition by synonym, by class, by description and so on). Examination of a large number of definitions from school children in Leeds suggested that it was impossible to group definitions with confidence in categories of this kind and that any attempt to assess their value should be based on criteria of a psychological kind.Definition by emotional tone appears to be one of the most rudimentary types of definition and though such definitions have frequently very little social value, they evidently occupy an important place in a psychological scale of values.Definition that associates meaning with a specific situation often appears superior to definition by emotional tone because it suggests a more socialised and mature concept: but within this category, many levels may be noted, ranging from the proto‐class to the purely emotional.Definitions that are generic in type, seem to reveal greater social maturity and would appear to be superior logically and psychologically to definitions in terms of specific contexts or emotional tone. Four kinds of definition of this type are distinguished.Various levels of definition are to be found within each of these main categories and it is suggested that the level of definition and its category should be considered separate criteria in any scale of semantic values. In general, the category indicates the extent to which the definition may be logically adequate and the level, the extent to which it reflects maturity of understanding. A good definition is satisfactory by either criterion but any definition that is less complete than this, should presumably, be judged by a criterion of conceptual as well as logical adequacy and in many situations (e.g., psychological tests), the former may be more useful than the latte