atrophy

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

• noun A wasting or decrease in size of a body organ, tissue, or part owing to disease, injury, or lack of use.
• noun A wasting away, deterioration, or diminution.
• intransitive verb To cause to wither or deteriorate; affect with atrophy.
• intransitive verb To waste away; wither or deteriorate.

from The Century Dictionary.

• To waste away.
• noun A wasting of the body, or of a part of it, owing to defective nutrition.
• noun In botany and zoology, arrested development of an organ due to stoppage of growth at any stage by the operation of causes either external to or inherent in the organism.

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

• intransitive verb To waste away; to dwindle.
• transitive verb To cause to waste away or become abortive; to starve or weaken.
• noun A wasting away from lack of nourishment; diminution in bulk or slow emaciation of the body or of any part.

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

• noun pathology A reduction in the functionality of an organ caused by disease, injury or lack of use.
• verb To wither or waste away.

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

• noun a decrease in size of an organ caused by disease or disuse
• noun any weakening or degeneration (especially through lack of use)
• verb undergo atrophy