The American Heritage® Dictionary of the English Language, 4th Edition
n. A meaningful linguistic unit consisting of a word, such as man, or a word element, such as -ed in walked, that cannot be divided into smaller meaningful parts.
n. The smallest linguistic unit within a word that can carry a meaning, such as "un-", "break", and "-able" in the word "unbreakable".
the GNU version of the Collaborative International Dictionary of English
n. The smallest unit of meaning of a language, which cannot be divided into smaller parts carrying meaning; it is usually smaller than a single wordform, such as the -ed morpheme of verbs in the past tense or the -s morpheme of nouns in the plural form.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. minimal meaningful language unit; it cannot be divided into smaller meaningful units
Word Usage
"Logograms are like Chinese characters: each represents a word or morpheme a morpheme is the smallest part of the word that has a meaning of its own."