Latin has no article. The bare noun could be used in contexts which in the Romance language require a definite or indefinite article. Modern Castilian has an indefinite article un, una, with masculine and feminine forms. Its origin is the Latin numeral unus ‘one’ ().

Apart from questionable exceptions, the numeral unus is not used as an indefinite article. Thus, there is no article or numeral on the noun in .

Only in colloquial and Late Latin did unus start to occur without any emphasis on cardinality ().

While a nominal modified by a numeral may receive definite determination, it is semantically indefinite without such determination. The desemanticization of the numeral to an indefinite article implies that the feature specifying the cardinality ceases to be contrastive, i.e. no contrast with a different cardinality is implied, while the indefinite feature remains.

In Old Castilian, the indefinite article has the two gender forms un y una, while the case declension is lost. It is used for specific indefinite reference to a singular entity. From the 16th century on, it may also precede a proper name, which then is used in a non-proper sense, like un Cicerón ‘a Cicero’, meaning ‘a great orator’. Likewise in early Modern Castilian, the indefinite article starts being used in non-specific reference ().

In Ibero-Romance (with traces in other Romance languages), the new indefinite article is also used in its plural form for reference to an indefinite plural set. First occurrences in Old Castilian are from the 13th century (); a contemporary example is in another section.

Thus, the cardinality feature of the indefinite article is lost altogether, and what remains is indefiniteness. The indefinite article forms a paradigm with the definite article in the initial position of the noun phrase; and the choice between them becomes obligatory in ever more contexts.

Reference

Pountain, Christopher J. 2019, ‘The development of the articles in Castilian: a functional approach’. Languages 4(2), 20 [online].