What is the medical definition of a fetus versus an embryo?

After fertilization: an ovum (egg) becomes a single-celled embryo. At later stages it is called a fetus - up until birth. When exactly an embryo becomes a fetus is somewhat arbitrary since development occurs gradually, but often is listed as 8 weeks post fertilization. Embryos look primitive, have incomplete organ development and are sensitive to teratogens; whereas a fetus is generally recognizable as being human.