In morphology, a bound morpheme is a morpheme (the most basic unit of meaning) that can appear only as part of a larger word; a free morpheme or unbound morpheme is one that can stand alone or can appear with other morphemes in a lexeme. A bound morpheme is also known as a bound form, and similarly a free morpheme is a free form.
Many roots are free morphemes (ship- in "shipment"), and others are bound. Roots normally carry lexical meaning. Words like chairman that contain two free morphemes (chair and man) are referred to as compound words.
Affixes are always bound in English, but some languages like Arabic have forms that sometimes affix to words and sometimes stand alone. English language affixes are almost exclusively prefixes or suffixes: pre- in "precaution" and -ment in "shipment". Affixes may be inflectional, indicating how a certain word relates to other words in a larger phrase, or derivational, changing either the part of speech or the actual meaning of a word.
Cranberry morphemes are a special form of bound morpheme whose independent meaning has been displaced and serves only to distinguish one word from another, like in cranberry, in which the free morpheme berry is preceded by the bound morpheme cran-, meaning "crane" from the earlier name for the berry, "crane berry".
Words can be formed purely from bound morphemes, as in English permit, ultimately from Latin per "through" + mittō "I send", where per- and -mit are bound morphemes in English. However, they are often thought of as simply a single morpheme.
A similar example is given in Chinese; most of its morphemes are monosyllabic and identified with a Chinese character because of the largely morphosyllabic script, but disyllabic words exist that cannot be analyzed into independent morphemes, such as 蝴蝶 húdié 'butterfly'. Then, the individual syllables and corresponding characters are used only in that word, and while they can be interpreted as bound morphemes 蝴 hú- and 蝶 -dié, it is more commonly considered a single disyllabic morpheme. See polysyllabic Chinese morphemes for further discussion.
Linguists usually distinguish between productive and unproductive forms when speaking about morphemes. For example, the morpheme ten- in tenant was originally derived from the Latin word tenere, "to hold", and the same basic meaning is seen in such words as "tenable" and "intention." But as ten- is not used in English to form new words, most linguists would not consider it to be a morpheme at all.
A language with a very low ratio of morphemes to words is an isolating language. Since such a language uses few bound morphemes, it expresses most grammatical relationships by word order or helper words, so it is an analytic language.
In contrast, a language that uses a substantial number of bound morphemes to express grammatical relationships is a synthetic language.