(1) “Survivor” means the eligible surviving spouse or eligible child or children of a member, deferred vested member, or retirant.

  (2) “Eligible child” means an unmarried child of a member, deferred vested member, or retirant who is:

  (a) Under 18 years of age.

  (b) Over 18 years of age with a mental or physical disability that precludes engaging in any gainful occupation.

  (c) Over 18 years of age and regularly attending high school or an accredited institution of higher learning until becoming 25 years of age or no longer regularly attending school, whichever first occurs.

  (3) “Surviving spouse” means the person to whom a member, deferred vested member, or retirant is legally married at the time of his or her death.