ri·bo·some (r??b?-s?m?)n. A structure composed of RNA and protein, present in large numbers in the cytoplasm of living cells and serving as the site for assembly of polypeptides encoded by messenger RNA.[ribo(nucleic acid) + -some.]ri?bo·so?mal (-s??m?l) adj.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.ribosome (?ra?b??s??m) n (Biochemistry) any of numerous minute particles in the cytoplasm of cells, either free or attached to the endoplasmic reticulum, that contain RNA and protein and are the site of protein synthesis[C20: from ribo(nucleic acid) + -some3] ?ribo?somal adjCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014ri?bo?some (?ra? b??so?m) n. a tiny, mitten-shaped organelle occurring in great numbers in the cell cytoplasm and functioning as the site of protein manufacture. [1958] ri`bo?so?mal, adj. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.ri·bo·some (r??b?-s?m?) A sphere-shaped structure within the cytoplasm of a cell that is composed of RNA and protein and is the site of protein synthesis. Ribosomes are often attached to the membrane of the endoplasmic reticulum. See more at cell.The American Heritage® Student Science Dictionary, Second Edition. Copyright © 2014 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.