A newborn is a child who has only just been born — hours or days old, not months. Doctors, parents, and news stories all use it. You can say a newborn baby (here it tells you what kind of baby) or a newborn (the baby itself). People also use it for animals: a newborn lamb, a newborn puppy. It is a warm, ordinary word, not slang and not fancy. It does not mean “new” the way a new phone is new; it is tied to coming into the world.