What is the heart valve in back of the heart called?

Mitral valve. The mitral valve is the most posterior and also the most leftward of the four heart valves. http://www.vhlab.umn.edu/atlas/anatutorial/anatutorial5.shtml.
Left side of heart. The heart is a pump made up of four chamber, the mitral valve is made up of two cusps and is located between the top left chamber (left atrium) and bottom left chamber (left ventricle). It is the heart valve closest to the spine.