a rare childhood neurological disorder of unknown cause that is characterized by the sudden or gradual development of aphasia (inability to understand or express language) and an abnormal electroencephalogram (see electroencephalography). The syndrome usually occurs in children between the ages of 5 and 7 years who develop normally but then lose their language skills for no apparent reason. Many children with the disorder experience seizures, which generally disappear by adulthood. [first described in 1957 by William M. Landau and Frank R. Kleffner]