The Collaborative International Dictionary of English v.0.48:

Veteran \Vet"er*an\ (v[e^]t"[~e]r*an), a. [L. veteranus, from
   vetus, veteris, old; akin to Gr. 'e`tos year, Skr. vatsara.
   See Wether.]
   Long exercised in anything, especially in military life and
   the duties of a soldier; long practiced or experienced; as, a
   veteran officer or soldier; veteran skill.
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         The insinuating eloquence and delicate flattery of
         veteran diplomatists and courtiers.      --Macaulay.
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The Collaborative International Dictionary of English v.0.48:

Veteran \Vet"er*an\ (v[e^]t"[~e]r*an), n. [L. veteranus (sc.
   miles): cf. F. v['e]t['e]ran.]
   One who has been long exercised in any service or art,
   particularly in war; one who has had much experience, or has
   grown old or decrepit in service.
   [1913 Webster]

         Ensigns that pierced the foe's remotest lines,
         The hardy veteran with tears resigns.    --Addison.
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   Note: In the United States, during the civil war, soldiers
         who had served through one term of enlistment and had
         reenlisted were specifically designated veterans.
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WordNet (r) 3.0 (2006):

veteran
    adj 1: rendered competent through trial and experience; "a
           seasoned traveler"; "veteran steadiness"; "a veteran
           officer" [syn: seasoned, veteran(a)]
    n 1: a serviceman who has seen considerable active service; "the
         veterans laughed at the new recruits" [syn: veteran,
         veteran soldier]
    2: a person who has served in the armed forces [syn: veteran,
       vet, ex-serviceman]
    3: an experienced person who has been through many battles;
       someone who has given long service [syn: veteran, old-
       timer, oldtimer, old hand, warhorse, old stager,
       stager]