Also means
a, an
Usage Note
Un is both the cardinal number 'one' (masculine) and the indefinite article 'a/an' (un libro, 'a book'). Before a feminine noun beginning with a stressed a- or ha-, the article is also un — not una — giving un agua fría, un hacha; this is purely phonological and the noun remains grammatically feminine. When used as a numeral before a masculine noun, un replaces uno (un millón).
Examples
"Necesito un momento para pensar."
Natural Translation
I need a moment to think.

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