combining form
Definition - A modified version of a word that combines with either other words or other combining forms to create new words.

Example -

Some example combining forms are

(1) the electro in electromagnet (from electric),

(2) the crypto in cryptography (from the Greek kryptos, hidden), and

(3) the geo in geography (from the Greek ge, earth).

Etymology -
The term was coined by the OED's editors in the 1880s for use in the OED.

Oxford English Dictionary -
Its first citation is from 1884, where the OED cites itself. The term's first non-OED citation is from 1942:
"In Latin and other languages, many words have a special combining form which appears only in compounds (or only in compounds and derivatives) …."
(Bloch & Trager Outl. Ling. Analysis 66)

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