What structure separates the left ventricle from the great vessel leaving the heart on the left side?

Aortic valve separates the left ventricle from the great vessel (aorta) leaving the heart on the left side.

Explanation:

Normally in heart, deoxygenated blood flows in the right atrium which then enters right ventricle through tricuspid and is pumped to the lungs by the pulmonary artery and from the lungs, the oxygenated blood enters left atrium and from here it enters left ventricle by passing through the mitral valve and from left ventricle it is pumped into aorta by the aortic valve.

Aortic valve is one of the two semilunar valves in the heart. One being the pulmonary valve present in the right heart and the second is the aortic valve.