1. Capable of life: A viable premature baby is one who is able to survive outside the womb.
2. Able to maintain an independent existence or able to live and develop: The viable seedlings were sown in the garden and were soon growing into big zucchini plants.
3. Eligible of success, or continuing effectiveness; practicable: The bank said that the plan for starting John's business was viable and could be carried out successfully.

Viable was originally restricted to the senses of "able to grow" and "able to survive", as in a viable fetus.

Its extended sense of "able to be done" or "worth doing", as in "viable alternatives", is now well established and acceptable in the English language.