combining form

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A modified form of independent word that occurs only in combination with words, affixes, or other combining forms to form compounds or derivatives, as electro- (from electric) in electromagnet or geo- (from Greek geō-, from gē "earth”) in geochemistry.

Wiktionary

1. n. A form of word used for combining with other words or other combining forms to make new words. A combining form may conjoin with an independent word (e.g., mini- + skirt), another combining form (e.g., photo- + -graphy) or an affix (e.g., cephal + -ic); it is thus distinguished from an affix, which can be added to either a free word or a combining form but not solely to another affix (e.g., Iceland + -ic but not pro- + -ic). It can also be distinguished historically from an affix when it is borrowed from another language in which it is descriptively a word (e.g., the French mal gave the English mal- in malodorous) or a combining form (e.g., the Greek kako-, a combining form of kakos, gave the English caco- in cacography).
2. n. In computer typography, the form of accent that can be combined with other characters, as opposed to a single character that includes the accent.

WordNet 3.0

1. n. a bound form used only in compounds