-th 1suff. Variant of -eth1.-th 2suff.1. Act; process: spilth.2. State; quality: dearth.[Middle English, from Old English -thu, n. suff.]-th 3 also -ethsuff. Used to form ordinal numbers: millionth.[Middle English -the, from Old English -tha, -the.]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.-th suffix forming nouns 1. (from verbs) indicating an action or its consequence: growth. 2. (from adjectives) indicating a quality: width. [from Old English -thu, -tho]-th or -ethsuffix (Mathematics) forming ordinal numbers: fourth; thousandth. [from Old English -( o)tha, -( o)the]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014Th Chem. Symbol. thorium. -th1 , a suffix forming nouns of action (birth) or abstract nouns denoting quality or condition (depth; length; warmth). [Middle English -th(e), Old English -thu, -tho, -th (variant -t after a velar, f, or s), c. Gothic -itha, Latin -tus, Greek -tos] -th2 , a suffix used in the formation of ordinal numbers: fourth; tenth. [Middle English -the, -te, Old English -tha, -the (variant -ta after f or s), c. Old Norse -thi, -di, Latin -tus, Greek -tos; compare -eth2] -th3 , var. of -eth1: doth. Th. Thursday. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.