"Homonymous hemianopsia" is the loss of one side of the field of vision in both eyes. It occurs when there is a lesion in the optic radiations which lie behind the optic chiasm, or damage to the occipital lobe on one side. Basically any damage to the optic tract behind the optic chiasm.

http://www.acbvi.org/albums/Vision/slide18.html

For longer slideshow on visual defects see http://www.acbvi.org/albums/Vision/

For a brief glossary on vision and visual defects see http://www.opt.indiana.edu/v514/lab6.htm