General: Edmund S. Conklin. The Definition of Introversion, Extroversion, and Allied Concepts. Journal of Abnormal Psychology and Social Psychology, Vol. XVII, No. 4, Jan.-Mar. 1923, p. 367.

A review of the concepts of introversion and extroversion according to Freud, White, Jung, Hinkle, and Wilfrid Lay (!). He defines extroversion as

a more or less prolonged condition in which attention is controlled by the objective conditions of attention more than by the subjective, and in which the content of the subjective conditions is most closely related to the objective.

Introversion is defined as

a more or less prolonged condition in which attention is controlled more by the subjective than by the objective conditions, and in which the content of the subjective conditions is of a more abstract nature and not so intimately related to the objective conditions.

There are, however, groups of people who do not conform to either of these two types;

such people are for a period thoroughly introverted, they are writing books or preparing lectures on some abstract subject, … and while so conform perfectly to my definition of introversion; at other times they are so completely absorbed in fishing, hunting, mountain climbing and tales of the same, … and while so conform perfectly to the definition given to extroversion.

These latter he calls ambiverts. There are in addition many other enlightening definitions.