Terminology and meaning 

ALIVE

adj. (usu. predic.) 1.(of a person, animal, plant, etc.) living, not dead. Oxford Dictionary 1990

Alive means that this being is growing, developing, maturing, and replacing its own dying cells. It means not being dead.

HUMAN

adj. & n. - adj. 1. of or belonging to the genus Homo.
Oxford Dictionary 1990

Homo Sapiens - unique from all other beings in that they have 46 human chromosomes in every cell. Such beings do not belong to any other family. 

FOETUS

n. an unborn or unhatched offspring of a mammal esp. a human one more than 8 weeks after conception.
Oxford Dictionary 1990

Latin for "young one," offspring and in the context of human persons, a young human person. But in recent times it has come to be understood in a very different, even an antithetical way, as something other than a person or a child. For some people it is a foetus instead of a child that a woman is carrying. It is psychologically easier to speak of the destruction of a foetus than of a child. Defenders of abortion object to the use of the term child - they generally do not want to appear as defending the killing of a child. So it is a foetus; destroying a foetus does not sound so bad. The reality of the child in the womb must be emphasised and conveyed to others. Suppose a woman suffers a miscarriage. A doctor will not tell her, "You have lost your foetus"; he will say, "You have lost your child." If the child is marked for destruction he may well be referred to as a foetus, for this term has a cold, scientific neutrality that effectively obscures the reality and preciousness of the small preborn child. 

VIABILITY

adj. b (of a foetus or newborn child) capable of maintaining life.
Oxford Dictionary 1990

Some people say that life begins when the unborn child reaches "viability", the point where he or she can survive outside the womb. When does a developing child become "viable? That is, what is the earliest a child could be born and still have a chance to survive? Forty years ago doctors put the age of viability at about 30 weeks into pregnancy, or ten weeks premature. Twenty years ago it was 25 weeks. Today it is 20 weeks. But viability used to be 30 weeks and is now 20. What's changed? Have babies changed? Have mothers changed? No, what's changed is the medical know-how of the doctors and the sophistication of the life support equipment available to them. Forty years ago doctors didn't have all the high-tech medical equipment they have today, so babies who would have died back then can now be saved. So what is "viability" measuring? It is a measure of the state of medical science and technology in a particular place at a particular time; it does not tell us anything about the baby.

QUICKENING

v. 3 intr. a (of a woman) reach a stage in pregnancy when movements of the foetus can be felt. b (of a foetus) begin to show signs of life.
Oxford Dictionary 1990

For several centuries the most popular belief was that life begins when the unborn child begins to move. This was called "quickening", which actually means "becoming alive". But back then they didn't have ways to see into the womb to really tell when a baby starts moving. By 18 weeks the baby can kick hard enough for the mother to feel it, but of course he's moving before then, though they weren't sure exactly when. Today we know that babies start moving at about six weeks.