cyst-pref. Variant of cysto-.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.cyst (s?st) n. 1. any abnormal saclike growth of the body in which matter is retained. 2. a bladder, sac, or vesicle. 3. a. a protective capsule or spore surrounding an inactive or resting organism or a reproductive body. b. such a capsule and its contents. [1705?15;