A bound morpheme is a morpheme that cannot stand alone and must be attached to another morpheme to form a meaningful word. There are several synonyms for bound morpheme, including affix, prefix, suffix, infix, and circumfix. An affix is a morpheme that can be added to the beginning or end of a word, such as the prefix "un-" or the suffix "-able." A prefix is an affix added to the beginning of a word, while a suffix is an affix added to the end of a word. An infix is a morpheme added within a word, while a circumfix is an affix added around a word. All of these terms describe the different types of bound morphemes used in language.