APA Dictionary of Psychology

the maximum (highest) level of complexity of a skill that an individual can control, which can be attained only in the most supportive environment. It has been suggested that when acquiring a skill an individual moves from the optimal level, at which the skill can be performed with assistance from others, to the functional level, at which the skill can be performed independently but possibly at a lower than optimal level. See skill theory. [proposed by U.S. psychologist Kurt W. Fischer (1943–  ]