This is a confusing topic. In Czech there is SLEPICE (adult female chicken) and KUŘE (baby to adolescent chicken of either sex). And of course, KOHOUT (adult male chicken = rooster).
Slepice is technically HEN
Kuře is technically CHICKEN and sometimes called CHICK.
So far so good. The confusing comes when you talk about groups of these. While we have SLEPICE in our back yard and use that word as a general blanket noun to describe all members of the chicken family, Americans (and probably Brits as well) have CHICKENS and that includes the adult animals. At the same time, when you buy chicken in the store, it still is called Chicken and it really is an adolescent bird. If you ever cooked adult "chicken" you would see the difference. They need to be cooked longer and are tougher. So confusingly enough Chicken is both baby chicken and adult, which is considered a hen in Czech.