an

The word 'an' is the direct phonetic and functional descendant of the Old English numeral 'ān', meaning "one." Its journey from a full numeral to an indefinite article is a classic example of grammaticalization, a process where a content word weakens into a function word. In Middle English, the vowel was lengthened in stressed positions, becoming the modern 'one', while the unstressed, prevocalic form 'an' lost its initial 'n' before consonants, giving us 'a'. Thus, 'an' preserves the original nasal sound before a vowel, maintaining its core logic of indicating a single, nonspecific instance of the noun it precedes.

This is a simplex word, a reduced form of the numeral 'one'. It originates from Old English 'ān' (one), from Proto-Germanic 'ainaz', from Proto-Indo-European 'óynos' (one, single).