A more thorough explanation:

An embryo is a developing but unborn or unhatched animal. In humans, it refers to the unborn human from conception until the development of organs, which is usually about the eighth week of pregnancy. It is different from a fetus, which is a developing human from the eighth week of pregnancy until birth.

There are two types of human embryos recognized in ecclesiastical law:

• Embryo formatus: A human embryo organized into human shape and endowed with a soul. This distinction was accepted by Gratian in his Decretum, where he stated that abortion is not murder if the fetus has not yet been infused with a soul. By the 16th century, canonists accepted that the time of formation and animation was the 40th day after conception for the male fetus and the 80th day for the female.
• Embryo informatus: A human embryo before it has been endowed with a soul.

For example, a human embryo at six weeks of gestation is considered an embryo because it has not yet developed all of its organs. However, a human fetus at 20 weeks of gestation is not an embryo because it has already developed all of its organs and is in the fetal stage of development.