Fertilization (British English spelling: fertilisation) is what happens when a female's ovum (or "egg") joins together with a male's sperm and they form a zygote. Fertilization is also called conception. Biologists call a fertilized egg a zygote. A zygote grows into an embryo.
Fertilization is important in animals including humans and birds, in plants, fungi, protists, in fact all eukaryotes. Fertilization makes a cell with twice as many chromosomes. The eukaryote life cycle must also include meiosis which divides the chromosome number in half.
In animals, there are two types of fertilisation, internal and external. Internal fertilisation happens in the female body. External fertilisation happens outside of the body. Mammals, birds, and reptiles use internal fertilisation. Amphibians and most fish use external fertilisation. Some animals with internal fertilization give birth to live offspring. Others such as birds, most reptiles, and some mammals such as the Platypus, lay eggs.
In humans, the zygote becomes an embryo when it reaches three weeks of age. It is called an embryo from 3–8 weeks (the embryonic period). After 8 weeks, it is called a fetus. If it continues to grow normally, it is called a baby as soon as it is born.