The Catholic Church is clear in its teaching on when life begins: at conception.
On death— described as “the end of man’s earthly pilgrimage”— the Church teaches in the Catechism of the Catholic Church that “life is changed, not ended;” that death represents the moment of “the separation of the soul from the body.”
While the moment of human conception— the beginning of life— is well-understood and observed from a scientific standpoint, the exact moment of death can be harder to pin down.
This is especially true thanks to various forms of modern technology such as ventilators, which make it possible for doctors to declare a patient dead based on the state of their brain, even if their body appears, to the untrained eye, still to be alive.
Brain death, also called death by neurological criteria, is the practice of declaring a person dead based on the loss of brain function, rather than the stoppage of the heart and breathing.