A female chicken older than one year is commonly called a hen.
In everyday poultry terminology, “hen” identifies an adult female chicken. A young female is generally called a pullet, although the exact age boundary can vary with breed and production context. An adult male is a rooster or cock, while a young male may be called a cockerel.
Hens can lay eggs without mating, but those unfertilized eggs cannot develop into chicks. A rooster is required for fertilization, not for the basic process of egg laying.
The word “hen” is also used more broadly for adult females of some other bird species, which can create ambiguity outside poultry discussions. In a chicken context, however, it specifically means the mature female bird.