First death: that of the body
Second death: separation from God.
First life: life of nature.
Second life: life of grace.
St. Thomas Aquinas, Compendium of the Summa
CHAPTER 239
THE TWOFOLD LIFE RESTORED IN MAN BY CHRIST
As Christ destroyed our death by His death, so He restored our life by His resurrection. Man has a twofold death and a twofold life. The first death is the death of the body, brought about by separation from the soul; the second death is brought about by separation from God. Christ, in whom the second death had no place, destroyed both of these deaths in us, that is, the bodily and the spiritual, by the first death He underwent, namely, that of the body.
Similarly, opposed to this twofold death, we are to understand that there is a twofold life. One is a life of the body, imparted by the soul, and this is called the life of nature. The other comes from God, and is called the life of justice or the life of grace. This life is given to us through faith, by which God dwells in us, according to Habakkuk 2:4: “The just shall live in his faith.”