indefinite article
Definition - Another name for the adjectives a and an and their equivalents in other languages.

1. It indicates a non-defined member of the species denoted by the noun (e.g. a black horse).

Etymology -
The term article derives from the Latin articulus, the diminutive of artus, a joint.

Oxford English Dictionary -
Its first citation is from 1727:
"The article a is said to be indefinite, because applied to names taken in their more general, and confused signification."
(Chambers Cycl. s.v. Article)

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