This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Linguistics. Please check back later for the full article.
Zero morphology or null morphology is the hypothesis that there are morphological units that are devoid of phonological content. This apparently simple situation is in fact one of the most controversial issues in morphological theory, with disagreements in how the concept should be delimited, what would count as an instance of zero morphology inside a particular theory, and whether such objects should even be allowed as mere analytical instruments
With respect to the first problem, given that zero morphology is a hypothesis that comes from certain analyses, delimiting what counts as a zero morpheme is not a trivial matter. The concept must be carefully differentiated from others that—intuitively—also involve situations where there is no overt morphological marking: cumulative morphology, phonological deletion, etc.
About the second issue, what counts as null can also depend on the specific theories where the proposal is made. In the strict sense, zero morphology involves a complete morphosyntactic representation that is associated to zero phonological content, but there are other notions of zero morphology that differ from here, such as absolute absence of morphological expression, in addition to specific theory-internal interpretations of what counts as null. Thus, it is also important to consider the different ways in which something can be morphologically silent
Finally, with respect to the third side of the debate, arguments are made for and against zero morphology, notably from the perspectives of falsifiability, acquisition, and psycholinguistics. Of particular impact is the question of which properties a theory should have in order to block the possibility that zero morphology exists, and—conversely—the properties that theories that accept zero morphology associate to null morphemes
An important ingredient in this debate has to do with two empirical domains: zero derivation and paradigmatic uniformity. Ultimately, the plausibility that zero morphemes exist or not depends on the success at accounting for such empirical patterns in a better way than theories that reject their existence, or define their assumptions in ways that zero morphology has no place