dac·ti·no·my·cin (d?k?t?-n?-m??s?n)n. An antibiotic of the actinomycin group, C62H86N12O16, isolated from bacteria and used as an antineoplastic agent in the treatment of certain cancers. Also called actinomycin D.[d- + actinomycin.]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.