air pocket

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

• noun An isolated space containing trapped air, as in bread dough or a collapsed mine shaft.
• noun A localized atmospheric condition, such as a downdraft, that causes an aircraft to lose altitude abruptly.

from the GNU version of the Collaborative International Dictionary of English.

• (aeronautics) A local region in the atmosphere having a downward movement and offering less than normal support for the sustaining surfaces of a flying machine, causing an airplane to drop suddenly. Same as air hole.

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

• noun a local region of low atmospheric pressure, or a local downward current; especially one that causes an aircraft to lose height suddenly

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

• noun a local region of low pressure or descending air that causes a plane to lose height suddenly