terrestrial planetn. Any of the four planets, Mercury, Venus, Earth, and Mars, whose orbits are closest to the sun. Also called inner planet.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.in?ner plan?et n. any of the four planets closest to the sun: Mercury, Venus, Earth, or Mars. [1950?55] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.