1. a combining form with the meaning “ray, beam,” used in the formation of compound words, with the particular senses “radiation” in the physical sciences (actinometer) and “having raylike structures,” “radiate in form” in biology (actinomyces; actinopod).
Also especially before a vowel, actin-.

Origin of actino-

< Greek, combining form representing aktī́s (genitive aktînos) ray, beam; see -o- Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for actino-


before a vowel actin-

combining form
  1. indicating a radial structureactinomorphic
  2. indicating radioactivity or radiationactinometer

Word Origin for actino-

from Greek aktino-, from aktis beam, ray
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for actino-

before vowels actin-, word-forming element meaning "pertaining to rays," from Greek aktis (genitive aktinos) "ray, radiance;" perhaps cognate with Sanskrit aktuh "light, ray," Gothic uhtwo "dawn, daybreak," Lithuanian anksti "early."

Online Etymology Dictionary, © 2010 Douglas Harper

actino- in Medicine


  1. Actinic radiation:actinotherapy.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.