Myelin, from the Greek myelos, marrow.
Myelin was discovered and the word coined in 1854 by the great German pathologist Rudolph Virchow (1821-1902). His investigations revealed that a fatty substance apparently identical to what could be found in the brain and spinal cord was also in every non-CNS organ he examined. His inspiration for the name of this material was the "nerve marrow" as it was called at the time: the white matter of the spinal cord seen within the spinal column's vertebral canal (and so named because of its resemblance to bone marrow within the medullary cavities of long bones).
Photo from the National Institutes of Health via Wikipedia
Virchow was one of the first to propose that disease occurs at the cellular level ("think microscopically" he frequently told his students). He was an early advocate of the scientific methodology in medical research and using such tools elucidated the mechanism of pulmonary thromboembolism, coining the words "thrombus" and "embolism" in the process. He fundamentally rejected the concept that some type of magical "vital principle" was necessary for living systems, a mechanistic philosophy radical for his times.
He was also social iconoclast, campaigning for the rights of the downtrodden and disenfranchised in oppressive Bismarck-era Prussia. Much of his political motivations stemmed from his unbending belief that disease was as much due to dire social factors as it was to those biological, and that democratization of society was the best way to improve the health of the poor.
In 1848 he was active in a failed revolution against the government, escaping punishment only because of strong support from his peers. In 1849, in large part because of this support, he became a professor at the University of Würzburg where five years later he did his work on myelin.
In 1902, at age 81, Virchow leaped from a moving streetcar, broke his hip, and died from complications.