Webster's English Dictionary

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an

1. an \*n, (')an\ ia [ME, fr. OE a-n one - more at ONE] : A - in standard 
   speech and writing used (1) invariably before w ords beginning with a vowel 
   letter and sound {an oak} (2) invariably before h-initial words in which 
   the h is silent {an honor}; (3) frequently before h-initial words which 
   have in an initial unstressed syllable an /h/ sound often lost after the an 
   {an historian} (4) sometimes esp. in England before words like union and 
   European whose initial letter is a vowel and whose initial sounds are /yu:/ 
   or /yu./
2. an or an' \see AND\ \(')an\ cj substand  1: AND  archaic  2: IF