A horizonn. In ABC soil, the uppermost, darkest zone that is rich in organic matter. Also called zone of eluviation, zone of leaching.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.A horizon n (Physical Geography) the top layer of a soil profile, usually dark-coloured and containing humus and from which soluble salts may have been leached. See B horizon, C horizon Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014A horizon n. the topsoil in a soil profile. [1935?40] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.