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.
In humans, biologists call a fertilized egg a zygote. It is called an embryo from 3–8 weeks (the embryonic period.) Then after 8 weeks, it is called a fetus. If it continues to grow normally it can eventually become a baby.
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 and birds use internal fertilisation. Reptiles, amphibians and fish use external fertilisation.