What is "buffy coat"

Buffy \Buff"y\, a. (Med.) Resembling, or characterized by, buff.

Buffy coat, the coagulated plasma of blood when the red corpuscles have so settled out that the coagulum appears nearly colorless. This is common in diseased conditions where the corpuscles run together more rapidly and in denser masses than usual.
--Huxley. [1913 Webster] ||

n. The fraction of an anticoagulated blood sample that contains most of the white blood cells and platelets following density gradient centrifugation of the blood.

The buffy coat is the fraction of an anticoagulated blood sample that contains most of the white blood cells and platelets following density gradient centrifugation of the blood.