The Collaborative International Dictionary of English v.0.48:

An \An\, conj. [Shortened fr. and, OE. an., and, sometimes and
   if, in introducing conditional clauses, like Icel. enda if,
   the same word as and. Prob. and was originally pleonastic
   before the conditional clause.]
   If; -- a word used by old English authors. --Shak.
   [1913 Webster]

         Nay, an thou dalliest, then I am thy foe. --B. Jonson.
   [1913 Webster]

   An if, and if; if.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

An \An\ ([a^]n). [AS. [=a]n one, the same word as the numeral.
   See One, and cf. A.]
   This word is properly an adjective, but is commonly called
   the indefinite article. It is used before nouns of the
   singular number only, and signifies one, or any, but somewhat
   less emphatically. In such expressions as "twice an hour,"
   "once an age," a shilling an ounce (see 2d A, 2), it has a
   distributive force, and is equivalent to each, every.
   [1913 Webster]

   Note: An is used before a word beginning with a vowel sound;
         as, an enemy, an hour. It in also often used before h
         sounded, when the accent of the word falls on the
         second syllable; as, an historian, an hyena, an heroic
         deed. Many writers use a before h in such positions.
         Anciently an was used before consonants as well as
         vowels.
         [1913 Webster]

WordNet (r) 3.0 (2006):

AN
    n 1: an associate degree in nursing [syn: Associate in
         Nursing, AN]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

AN
       Access Node


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

AN
       Alternating Network


The Free On-line Dictionary of Computing (30 December 2018):

an

    The country code for the Netherlands Antilles
   (Dutch Antilles).

   (1999-01-27)


Bouvier's Law Dictionary, Revised 6th Ed (1856):

AN, JOUR, ET WASTE. See Year, day, and waste.