Title 38 of the Code of Federal Regulations defines a veteran as such: 

Veteran means a person who served in the active military, naval, air, or space service and who was discharged or released under conditions other than dishonorable.

(1) For compensation and dependency and indemnity compensation the term veteran includes a person who died in active service and whose death was not due to willful misconduct.

(2) For death pension the term veteran includes a person who died in active service under conditions which preclude payment of service-connected death benefits, provided such person had completed at least 2 years honorable military, naval, air, or space service, as certified by the Secretary concerned. (See §§ 3.3(b)(3)(i) and 3.3(b)(4)(i))

(Authority: 38 U.S.C. 501)

(e) Veteran of any war means any veteran who served in the active military, naval, air, or space service during a period of war as set forth in § 3.2.

Title 38 Full Text