The Collaborative International Dictionary of English v.0.48:

Viable \Vi"a*ble\, a. [F., from vie life, L. vita. See Vital.]
   (Law)
   Capable of living; born alive and with such form and
   development of organs as to be capable of living; -- said of
   a newborn, or a prematurely born, infant.
   [1913 Webster]

         VIABLE, Vitae habilis, capable of living. This is said
         of a child who is born alive in such an advanced state
         of formation as to be capable of living. Unless be is
         born viable he acquires no rights and cannot transmit
         them to his heirs, and is considered as if he had never
         been born.                               --Bouvier (Law
                                                  Dictionary,
                                                  1856).
   [1913 Webster]

WordNet (r) 3.0 (2006):

viable
    adj 1: capable of being done with means at hand and
           circumstances as they are [syn: feasible, executable,
           practicable, viable, workable]
    2: capable of life or normal growth and development; "viable
       seeds"; "a viable fetus"

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "viable":
   OK, aboveground, acceptable, achievable, actable, activating,
   admissible, agreeable, alive, alive and kicking, all right,
   alright, among the living, animate, animated, animating, animative,
   applicable, attainable, bracing, breathing, capable of life,
   compassable, conscious, doable, endowed with life, energizing,
   enlivened, enlivening, exhilarating, exhilarative, existent,
   feasible, good enough, in the flesh, inspirited,
   instinct with life, invigorating, invigorative, live, living,
   long-lived, manageable, maneuverable, manipulatable, negotiable,
   of concern, of concernment, of consequence, of interest, of moment,
   of note, of significance, of weight, okay, operable, operatable,
   overcomable, passable, performable, possible, practicable,
   practical, quick, quickening, realizable, reasonable, rousing,
   sensible, stimulating, stimulative, superable, supportable,
   surmountable, sustainable, tenable, tenacious of life, tonic,
   unexceptionable, unobjectionable, very much alive, vital,
   vitalizing, vivified, workable, zoetic



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

VIABLE, Vitae habilis, capable of living. This is said of a child who is
born alive in such an advanced state of formation as to be capable of
living. Unless be is born viable he acquires no rights and cannot transmit
them to his heirs, and is considered as if he bad never been born.
     2. This term is used In the French law, Toull. Dr. Civ. Fr. tome 4, p.
101 it would be well to engraft it on our own Vide Traill. Med. Jur. 46, and
Dead Born.