Real Dictionary

combining form
countable noun: 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 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)
noun: a linguistic form that occurs only in combination with other forms. In word formation, a combining form may conjoin with an independent word (mini- + skirt), another combining form (photo- + -graphy), or an affix (cephal- + -ic); it is thus distinct from

combining form in British English
noun: a linguistic element that occurs only as part of a compound word, such as anthropo- in anthropology