Akachan (赤ちゃん, あかちゃん) means “baby.”
The word is a diminutive: aka (赤, あか) is “red,” and -chan (ちゃん) is an affectionate suffix. So akachan literally amounts to something like “little red one” – a reference to the flushed skin of a newborn. Akachan is the ordinary, everyday word.
It is also worth noting that -chan is not limited to babies. It attaches to names and nouns to mark endearment or smallness, and it can be used for children, close friends, and even adults in familiar speech.