Lexis meaning ‘word, phrase’ (hence lexicos ‘having to do with words’) and logos

2. the polisemy degree criterion
Derived words are usually less polysemantic than the simple ones used as their sources. A lower degree of polysemy of a word in a pair may be regarded as an indicator of its derived character.
3. Non-semantic criterion. Sinonimity criterion
The criterion is based on a comparison of a conversial pair with a synonymic word pair.A comparison of a conversion pair with analogous word-pairs making use of the synonymic sets, of which the words in question are members. For instance, in comparing conversion pairs like chat v — chat n; show v — show n; work v — work n, etc. with analogous synonymic word-pairs like converse — conversation; exhibit — exhibition; occupy — occupation; employ — employment, etc. we are led to conclude that the nouns chat, show, work, etc. are the derived.
4. The frequency criterion
Lower frequency value of a word in a conversial pair indicates its derived character.
To give an illustration, according to M. West’s A General Service List of English Words, the frequency value of four verb — noun conversion pairs in correlative meanings taken at random is estimated as follows:
to answer (V = 63%) — answer (N =35%), to help (V = 61%) — help (N = 1%), to sample (V= 10%) — sample (N=90%), to joke (V=8%) — joke (N=82%).

Conversion is highly productive in the formation of verbs, especially from compound nouns. In the 20th century new words include a great many verbs formed by conversion, e.g. to motor — ‘travel by car’; to phone — ‘use the telephone’; to wire — ’send a telegram’; to microfilm — ‘produce a microfilm of; to tear-gas — ‘to use tear-gas’. A diachronic survey of the present-day stock of conversion pairs reveals, however, that not all of them have been created on the semantic patterns just referred to. Some of them arose as a result of the disappearance of inflections in the course of the historical development of the English language due to which two words of different parts of speech, e.g. a verb and a noun, coincided in pronunciation. This is the case with such word-pairs, for instance, as love n (OE. lufu) — love v (OE. lufian).

On the diachronic plane conversion is a way of forming new words on the analogy of the semantic patterns available in the language. Diachronically distinction should be made between cases of conversion as such and those of homonymy due to the disappearance of inflections in the course of the development of the English language.