I don’t know if we have a definition when she is consider a cow and not a heifer, but this is my classification: A female bovine at birth is called a heifer calf. If she is retained as a potential female that will be part of the cow herd, she is called a replacement heifer. After she has given birth to her first calf — usually at about 2-years of age — she is called a cow.
Webster’s definition: a fully grown female animal of the ox family, usually kept in herds by farmers for milk … the female of any animal whose male is known as a bull, for example, the elephant, rhinoceros, whale, seal, moose.