The document defines key terms used in embryonic development: zygote refers to the single cell formed from the union of an ovum and sperm; embryo describes an organism in early development before distinct features form; fetus is used once major organs are present, around the 8th week in humans; fertilization is the union of sex cells during reproduction, called insemination in humans.

1. Power Words in Embryonic Development
Zygote- the cell formed from the union of male and female sex cells
(ovum and sperm).
Embryo- An organism in the early stages of its development, before it
has reached a distinctly recognizable form.
Fetus- The unborn, young vertebrate having a basic structural
resemblance to the adult animal. In humans, the embryo becomes a
fetus at the end of the 8th week of development, when all major
organs are present.
Fertilization- The union of male and female sex cells during sexual
reproduction. In plants, this process is called pollination and in
humans it is called insemination.