In other words it denotes potential. This means that even a seed that lies dormant for months or years is still considered viable so long as it retains this ability. Blackmun chose to assume that viability, or potential, and vitality, the actual presence of life could be considered separately and exclusively. This, in fact, cannot be done. While life may be present in the absence of viability, viability cannot exist much less be achieved in the absence of life, or vitality.
Viability is a condition and a property of vitality.
One cannot posses or achieve viability in the absence of vitality. Potential is meaningless in the absence of a potentiator. Viability is, at best, an indicator of the disposition of life rather than the determinate of life. However, this did not deter Blackmun. The ability of the pre-partum fetus to grow and develop within the womb qualifies it, by definition, as viable and therefore vital or alive.