Formal:
* 婴儿 (yīng'ér) - This is the most common and formal term for "baby," literally meaning "infant." It is used for babies of all ages.
Informal:
* 宝宝 (bǎo'bao) - This is a common and affectionate term for "baby," used for babies and toddlers. It's like saying "little treasure."
* 宝贝 (bǎo'bèi) - This is similar to "宝宝" but slightly more affectionate and can be used for children, pets, or even significant others.
* 小宝贝 (xiǎo bǎo'bèi) - This is even more affectionate than "宝贝" and can be used for young children.
Other options:
* 孩子 (hái'zi) - This literally means "child" and can be used for babies, toddlers, and older children.
* 小家伙 (xiǎo jiā'huo) - This literally means "little fellow" and can be used for babies and toddlers.
The best way to choose the right term is to consider the context and the relationship you have with the person you're speaking to.
For example, you would use "婴儿" to refer to a baby in a medical context, but you would use "宝宝" or "宝贝" to talk about your own baby.