aer·o·al·ler·gen (âr??-?l??r-j?n)n. Any of various airborne substances, such as pollen or spores, that can cause an allergic response.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.