The noun viability means the quality of being able to happen or having a reasonable chance of success. The viability of holding your party at a restaurant might depend on how many guests they can seat.
Viability comes from the Latin root vita, meaning "life." So the noun viability also refers to something's capacity to live and grow. When your dog has a litter of puppies, you can't help wondering about the viability of the tiniest one, but if you feed him with an eyedropper and keep him warm, he might make it.