Mitral is derived from the Latin mitra, girdle or turban, which gave rise to mitre, the ceremonial head-dress first worn by Roman Catholic bishops. Subsequently the hat was adopted by bishops of the Anglican, Episcopal, and some of the Christian orthodox churches.
A 15th century bishops' mitre on the left, a present-day mitre on the right, and in the middle, a 16th century-style mitre from the time of Vesalius.
Adapted from the Catholic Encyclopedia www.newadvent.org
The mitral valve of the heart, positioned between the left atrium and left ventricle, was so named by Vesalius in the 16th century because its two parallel cusps (flaps) resemble of the paired, pointed "horns" or cornua of an (upside-down) bishop's mitre. Thomas Henry Huxley, "Darwin's bulldog" and the person who coined "agnostic", would give his students this mnemonic to remind them the valve was on the left: "a bishop is never in the right."
The mitral valve is also known as the bicuspid valve or the left atrioventricular (AV) valve. The Terminologia Anatomica no longer lists bicuspid.