suffix

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An affix added to the end of a word or stem, serving to form a new word or functioning as an inflectional ending, such as -ness in gentleness, -ing in walking, or -s in sits.
• transitive verb To add as a suffix.

from The Century Dictionary.

• To attach at the end: specifically used of adding or annexing a letter or syllable, a suffix.
• noun In. grammar, a letter or syllable added or annexed to the end of a word or to a verbal root or stem; a formative element, consisting of one or more letters, added to a primitive word to make a derivative; a postfix; a terminal formative, as the -th of length, the -d of loved, the -ly of godly, the -ly of badly, etc.
• noun In mathematics, an index written after and under a letter, as x₀, x₁, x₂, x₃.

from the GNU version of the Collaborative International Dictionary of English.

• transitive verb To add or annex to the end, as a letter or syllable to a word; to append.
• noun A letter, letters, syllable, or syllables added or appended to the end of a word or a root to modify the meaning; a postfix.
• noun (Math.) A subscript mark, number, or letter. See Subscript, a.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun One or more letters or sounds added at the end of a word to modify the word's meaning, such as able, which changes sing into singable, for example.
• verb transitive to append (something) to the end of something else

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb attach a suffix to
• noun an affix that is added at the end of the word