Pho·bos (f??b?s)n. The larger and inner of the two planetary satellites of Mars.[Greek, fear, deity who personifies fear, from phobos, fear; see -phobe.]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.Phobos (?f??b?s) n (Celestial Objects) the larger of the two satellites of Mars and the closer to the planet. Approximate diameter (although it has an irregular shape): 23 km. Compare DeimosCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014Pho?bos (?fo? b?s, -b?s) n. one of the two moons of Mars. [