blind spot

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Anatomy The small, circular, optically insensitive region in the retina where fibers of the optic nerve emerge from the eyeball. It has no rods or cones.
• noun An area that one cannot see because of an obstruction.
• noun An area where radio reception is weak or nonexistent.
• noun A subject about which one is markedly ignorant or prejudiced.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The place where the optic nerve attaches to the retina, and so where the retina cannot detect light.
• noun In driving, the part of the road that cannot be seen in the rear-view mirror.
• noun figuratively An inability to recognize a fact or think clearly about a certain topic, especially because of a prejudice.
• noun A location where radio reception and/or transmission is significantly poorer than in surrounding locations.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the point where the optic nerve enters the retina; not sensitive to light
• noun a subject about which you are ignorant or prejudiced and fail to exercise good judgment