It follows from this definition that the equality of the real wage to the marginal disutility of employment presupposed by the second postulate, realistically interpreted, corresponds to the absence of 'involuntary' unemployment. This state of affairs we shall describe as 'full' employment, both 'frictional' and 'voluntary' unemployment being consistent with 'full' employment thus defined.
Apparent unemployment must, therefore, be the result either of temporary loss of work of the 'between jobs' type or of intermittent demand for highly specialized resources or of the effect of a trade union 'closed shop' on the employment of free labor.
--J.M. Keynes, "The General Theory of Employment, Interest, and Money"