spinal muscular atrophyn. Any of several forms of a hereditary, progressive disease that involves loss of motor neurons in the spinal cord and brainstem and is characterized by muscle weakness and atrophy that vary in severity and age of onset.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.