Although cattle are often collectively referred to as cows, the word cow specifically refers to females of the cattle species that have given birth to at least one calf. Young female cattle that have not yet given birth to a calf are referred to as heifers.
The word calf is used to refer to young cattle of both sexes until the point at which they are weaned. After weaning, they are known as yearlings until they reach the age of 2. At that point, the females become known as heifers until they give birth and become known as cows. Male cattle ages 2 and older are known as bulls. If a male is castrated, it is called a steer.