Myelin and Myelin Sheath https://medlineplus.gov/ency/article/002261.htm

Why is Myelin used as a term to mean the Myelin Sheath as opposed to the proper term?

It is apparent to me that Myelin is the substance itself regardless of structure and the Myelin Sheath is the structure around the axis to insulate Electro-Chemically the nervous system composed of this substance.

But Media and Physicians frequently use Myelin instead of Myelin Sheath as opposed to textbooks that make the distinction
As Wikipedia points out:


  Myelin is a lipid-rich (fatty) substance formed in the central nervous system (CNS) by glial cells called oligodendrocytes, and in the peripheral nervous system (PNS) by Schwann cells.


When referring to the sheath, you are referring to the covering which is made of myelin.


  Myelin sheaths are sleeves of fatty tissue that protect your nerve cells from damage (WebMD).


The thing is that myelin formed in the CNS and PNS is only used in the form of a sheath  to protect your nerve cells, so when physicians refer to the myelin, they are referring to the sheath.

With demyelination, the integrity of the sheath is being compromised.