A combining form can be either a prefix or a suffix; the difference is that the combining form adds a layer of extra meaning to the word. For example, bio‑ adds the idea of life or living things to words, as in biochemistry, the study of the chemical processes which occur within living organisms; ‑cide adds the idea of killing or a killing agent, as in pesticide. Compare these examples with a prefix such as ex‑ or a suffix such as ‑ic, neither of which add meaning, but only modify an existing meaning.
Combining forms only appear as elements in a compound. If it can stand alone as a word it is not a combining form. For example, carbo‑ only appears in compounds to indicate carbon, but there are many related words that begin with carbon‑; these are considered to be compound words and carbon‑ is not listed on this site as a combining form. Having said that, in some cases a combining form has at some point in its life taken on the status of a free-standing word (cyber‑ is an example), but if its primary function is as a combining form, it appears in its place in the text.
To be a combining form an element must be found attached to stems that also have intrinsic meaning; this excludes stems whose only compounds are grammatical variations, such as intense (intensive, intensively, intensiveness).
An infix is placed within a word; these are rare in English, though cupful can be made plural as cupsful by inserting the plural s as an infix; infixes sometimes occur in facetious creations like absobloodylutely (which some grammarians would rather describe as tmesis). Infixes often appear as linking vowels between prefixes and stems, for example the final letters of narco‑ and calci‑. They are also found between a stem ending in a consonant and a suffix beginning with one, as with ‑ferous, which frequently appears as ‑iferous, or ‑logy, which is commonly seen as ‑ology. The only examples of such linking vowel infixes here are ‑i‑ and ‑o‑.
No formal identification is made in the text of the class of affix to which entries belong. The position of the hyphen is sufficient indication whether it is placed at the beginning, in the middle or at the end of a word: neo‑, ‑i‑, ‑graphy.
Many prefixes that end in a vowel can lose that vowel when attached to a stem that begins in one, as for example phlebo‑ loses its final letter in phlebitis. Such cases are marked by enclosing the final letter of the headword in parentheses: phleb(o)‑.
The term productive has a special sense throughout the site: it refers to an affix which is active in the language and which is being used by writers today to create new words.