A combining form is a word that is used, or used with a particular meaning, only when joined to another word. For example, '-legged' as in 'four-legged' and '-fold' as in 'fivefold' are combining forms.
combining form in British English
noun
a linguistic element that occurs only as part of a compound word, such as anthropo- in anthropology
combining form in American English
a word form that occurs only in compounds or derivatives, and that can combine with other such forms or with affixes to form a word (Ex.: bio- and -lysis in biolysis)