The definition of a diagnosis identifies explicitly which cases are to be recorded under that term and, by implication, which are to be specifically excluded. The definition is the basis for planning treatment and for counting cases in a population. Classification within a diagnosis categorizes those cases with similar characteristics together and distinguishes those cases with diverse features apart. The design of a classification system, for instance whether it is organized into nominal or ordinal categories, will vary depending on the concept being classified and intended purpose for which classification is being made. The most frequently cited definition of cerebral palsy was published by Bax (1964) as 'a disorder of posture and movement due to a defect or lesion in the immature brain'. The label does however encompass a variety of syndromes and some, therefore, prefer the term cerebral palsies.