Pluralia tantum
and
singularia tantum
are nouns that are, respectively, only plural or only singular. When considered as a single linguistic phenomenon rather than as a set of anomalies, these nouns reveal aspects that go beyond the simplicity associated with defective number paradigms. This entry offers an overview of the various empirical facets of this phenomenon, covering
pluralia tantum
and
singularia tantum
as well as examining aspects of number defectivity that concern other values, in particular the dual. An initial typology organizes the empirical landscape of plural‐only and singular‐only nouns and identifies what is systematic, common or irregular about them. This typology highlights some semantically based clusters, which point to a semantic motivation behind cross‐linguistically recurring notions; it brings out the partial asymmetry between
singularia
and
pluralia tantum
– a feature related to the greater predictability of the singular for mass nouns; and it places the few attested cases of dual and other values in this general context. The empirical overview paves the way for a deeper theoretical discussion, which (i) considers to what extent
pluralia
and
singularia tantum
may correlate with a choice of gender or of noun class; (ii) distinguishes the syntagmatic reflexes of the paradigmatic lack of a number value; and (iii) argues that number defectivity is best understood via a conceptualization that induces misalignment between the morphological, the morphosyntactic and the semantic characterizations of a noun. The final section concludes that, if at one level the lack of a number value is a grammatical property of some lexical choices, at a deeper level it reflects how a particular conceptualization finds linguistic expression in the grammar and content of a word. What looks like a list of lexical exceptions is, then, a coherent linguistic phenomenon.