It is defined as the direct killing of a human foetus or unborn child. It is forbidden by the Catholic Church.
The Church says that a baby should not be killed in the womb even when it is necessary to save the mother’s life for it is not an unjust aggressor. That this is motivated by malice is proven by the fact that the Church lets you kill insane people in self-defence. It permits you to kill an insane person who can’t be an unjust aggressor because he doesn’t know what he is doing.  The Church alleges that its teaching does no harm. Even if that were true, would you want a religion that would prefer to sacrifice the baby and the mother to the natural course just because abortion is supposed to be a sin to have power in society?