My pro-abortion friend argues that a fetus does not have a soul until after it is born. I’m having trouble explaining the term “soul”. Can you help me?

The glossary at the back of the U.S. version of the Catechism of the Catholic Church defines "soul" as follows:

The spiritual principle of human beings. The soul is the subject of human consciousness and freedom; soul and body together form one unique human nature. Each human soul is individual and immortal, immediately created by God. The soul does not die with the body, from which it is separated by death, and with which it will be reunited in the final resurrection.

Here’s more:

In Sacred Scripture the term "soul" often refers to human life or the entire human person. But "soul" also refers to the innermost aspect of man, that which is of greatest value in him, that by which he is most especially in God’s image: "Soul" signifies the spiritual principle in man. The human body shares in the dignity of "the image of God": it is a human body precisely because it is animated by a spiritual soul, and it is the whole human person that is intended to become, in the body of Christ, a temple of the Spirit. Man, though made of body and soul, is a unity (CCC 363-364).