White matter disorders are all disorders in which the white matter is only or predominantly involved. The name has no implications regarding heritability or tissue pathology. White matter disorders are opposed to gray matter disorders in which the gray matter is only or predominantly involved. There are also disorders that affect the brain in a non-selective fashion. These are neither gray nor white matter disorders.
Leukoencephalopathy is the medical word for white matter disorder.
Leukodystrophies are white matter disorders caused by a genetic defect (therefore: inherited) and pathologically characterized by progressive breakdown and loss of myelin. The term leukodystrophy is often used when an extensive white matter disease is seen on MRI, especially in children, but this is not correct. Not all extensive white matter disorders in children are inherited and not all are progressive.
Demyelination means loss of myelin. Some make a difference between demyelination and dysmyelination, and reserve the word dysmyelination for inherited conditions in which myelin is lost and the word demyelination for the acquired conditions in which myelin is lost. Others take a more literal interpretation of these words and reserve the term demyelination for the conditions in which myelin is lost (de- indicating a loss of something), while the term dysmyelination is used for conditions in which the process of myelination is abnormal and irregular (dys- indicating an abnormality of something).
Myelination is the process in which myelin sheaths are formed to surround nerve fibers. The process of myelination of the brain is highly organized, structures becoming myelinated at a certain time and in a certain order. Most of the process takes place during the first year of life. Retarded myelination indicates that the deposition of myelin is delayed, but progressing. Hypomyelination refers to a permanent significant deficit of myelin in the brain.