My Chambers dictionary also gives "combining element", but "combining form" is a more usual term. You could perhaps put "(neo)classical" in front of it if you want to stress the Latin/Greek aspect, but I don't think that's really necessary. Words formed from them are called "Classical compounds", but the "elemente" themselves are "combining forms".

combining form
n.

A modified form of an 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 ge-, from g "earth") in geochemistry.
http://www.thefreedictionary.com/combining form

Combining form

In the linguistics of word formation, a combining form (also neoclassical element) is a bound base designed to combine with another combining form or a free word. For example, bio- combines with -graphy to form biography.

Words with combining forms are variously called combining form compounds, neoclassical compounds, and classical compounds.
http://ged.insead.fr/definitions/Combining Form?cx=partner-p...

See also:
http://en.wikipedia.org/wiki/Classical_compound