Zero derivation (conversion) is word formation without overt morphology: a word changes part of speech through syntactic or pragmatic reanalysis. English examples: 'to run' (verb from noun), 'a run' (noun from verb), 'green' (verb, adjective, noun). Conversion is particularly productive in English and Germanic languages.
Your prerequisite study of word formation rules showed you how morphological processes like affixation create new words: adding -er to "teach" yields "teacher," adding un- to "happy" yields "unhappy." These processes are overt — you can see and hear the morpheme that was added. Zero derivation (also called conversion) is morphologically minimalist: the word class changes, but no affix is added, no sound changes, nothing is overtly modified. The form is identical; the grammatical category is new.
English is exceptionally productive for zero derivation because it has so little inflectional morphology — unlike Latin or Russian, where nouns and verbs carry distinct case and agreement endings that prevent them from being used interchangeably, English words often have the same surface form regardless of category. "Google" was a proper noun before it became a verb meaning "to search." "Text" was a noun before texting existed as a concept. "Google me" and "text her" are zero derivations: the noun form was reanalyzed syntactically as a verb without any phonological or morphological change. This reanalysis happens through context — placing a word in a verbal position (subject-verb-object structure, following an auxiliary, taking a past-tense reading) is enough to license its use as a verb.
Directionality is the tricky theoretical question in zero derivation: which form is the base, and which is the derived form? For "run," most analysts treat the verb as primary (it is older in English) and the noun ("a run in the park," "a run in her stockings") as derived. But directionality is often genuinely ambiguous and can only be resolved by historical evidence, frequency data, or semantic complexity. The noun "hammer" and the verb "to hammer" — which came first? In practice, the word-formation rule framework you've already learned can represent zero derivation as a rule with a zero morpheme (∅): Noun → Verb / ∅ in verbal context. The ∅ morpheme is an abstract placeholder that marks the category change without being realized phonologically.
Conversion interacts interestingly with morphological structure. Complex words derived by affixation can themselves undergo conversion: "to microwave" (verb from the compound noun), "to one-up" (verb from the numeral phrase), "to out-Herod Herod" (the classical example of a proper noun verbed through zero derivation, then prefixed). This shows that conversion is not just a root-level process — it applies to morphologically complex forms and interacts with the broader word-formation system. When you encounter an unfamiliar use of a known word in a new syntactic position, zero derivation is one of the first processes to consider: English speakers regularly and productively extend words across category boundaries when the context makes the meaning recoverable.