In contrast to reduplication which because of its nature is rather doubtful in being clearly labelled as a tool of non-concatenative morphology branch, conversion is an easily-defined word-formation process.
Conversion takes place when a given word changes its word-class, hence becoming a new one. Because this involves no extension at the level of the word's internal structure, this process is also called zero-derivation or zero-affixation. It is argued that even though there is no visual representation of meaning-derivation, the so-called zero morpheme is added to the base as a justification for the change: [cook]v → [[cook]V + O ]N.
What can be exposed to modification, however, is the way the word is stressed (compare 'import [n.] and im'port [v.]). Despite this being the only noticeable change, there are several ways to find out which one is the derivative of the other. These include the date of the word's first appearance, frequency of the word's occurrence in use and complexity of meaning. Because the word that appeared first tends to carry less content meaning-wise, the one whose meaning is extended is regarded to be a derivative. Although conversion seems to be a relatively free process and can produce derivatives of virtually any word-class , four certain types of this phenomenon seem to prevail in English : noun-to-verb (a pilot [n.] — to pilot [v.]), verb-to-noun (to cook [v.] — a cook [n.]), adjective-to-noun (professional [a.] — a professional [n.]) and adjective-to-verb (empty [v.] — to empty [v.]). Also, one can distinguish between partial conversion and total conversion. Whereas total conversion deals with the transformation of the word into a new one following all its principles of grammatical suitability, partial conversion derivatives can differ from the original in a way (eg. advice — to advise, house — to house: /s/ is exchanged for /z/).