An egg cell is a special cell made by female animals (including humans) that can join with a sperm cell from a male to create a new living thing. Scientists call it an ovum, and it's one of the largest cells in the human body, though it's still barely visible to the naked eye, about the size of a grain of sand.

Every person who has ever lived started as a single egg cell that joined with a sperm cell. Inside that tiny cell was all the information needed to grow into a complete human being. The egg cell contains half the genetic instructions (from the mother), and when it combines with a sperm cell (which carries half from the father), you get a complete set of instructions for building a new person.

Different animals produce egg cells in different ways. Birds lay eggs with hard shells that develop outside the mother's body. Mammals like humans keep their egg cells inside, where they can develop into babies in a protected environment. A chicken egg you might eat for breakfast comes from an unfertilized egg cell, meaning it never joined with a sperm cell, so it won't develop into a chick.

The term can also refer to plant reproduction, where egg cells combine with pollen to create seeds.