8-chlo·ro·the·oph·yl·line (?t?klô?r?-th?-?f??-l?n)n. A stimulant, C7H7N4O2Cl, that is used to form a salt with certain pharmaceutical drugs, such as diphenhydramine.[8-, conventional numbering of the position in the compound’s purine ring to which the chlorine atom is attached + chloro- + theophylline.]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.