Conversion is a process of word-formation which transfers a word of one word class into a different word class without any formal modification of its stem. Secondarily, the term may also apply, beyond word-formation, to the recategorization of higher-level constructions. It is, at any rate, a recategorization not signalled by any overt means, differing in this from category-changing derivation. Conversion of a word A which, as a member of category X, occurs in context B, into category Y by use in context C may be represented in the following formula:
[ A ]X B → [ A ]Y C
English has conversion from verb to noun and from noun to verb as shown in the following diagram:
| noun | direction | verb |
|---|---|---|
| (the) run | ← | (to) run |
| (the) butter | → | (to) butter |
As in any instance of word formation, there is a creative phase in which speakers produce a new word on the basis of an existent lexical item by applying an operation to it. This may be followed by an inventorying phase in which the product of the first phase is lexicalized, so speakers may take a holistic access to it. Then conversion is completed, and the result is multiple categoriality of the stem in question.
The first phase of conversion avails itself of the mechanism of coercion. Coercion is a mechanism of recategorization which inserts the expression to be recategorized in a context which only allows members of the target category. This may take different forms depending on the grammatical level of the context. In English, the conversion of verb to noun (substantivization) involves use of the item in question in a context like a/the __, e.g. a run. Here the coercing context is a syntactic context. The conversion from noun to verb (verbalization) involves use of the item in question in a context like __-ed / to __, e.g. buttered a bread. This is a morphological context. Observe that this is not derivation of a verb from a noun, as there is no derivational operator (there is no need for the concept ‘zero derivation’); the stem – originally a noun stem – is just being conjugated.
Accepting a distinction between major and minor word classes, we can see that English even allows for conversion of items of a minor into a major class. illustrates substantivization of conjunctions and adverbs; illustrates verbalization of adverbs.
| . | a. | the ifs and buts |
| b. | an out |
| . | a. | to down a beer |
| b. | to out oneself |
We will come back to such examples in the section on degrammaticalization. At this point, it suffices to note two points:
- The process of conversion as such is not intrinsically oriented; i.e., it does not exclusively lead from major to minor classes, or from verbs to nouns. The source and the target category may be any.
- Conversion is a kind of word formation, thus having nothing to do with grammaticalization.
On account of the first observation, conversion has occasionally been called – though somewhat pleonastically – lateral conversion.