Formal:
* शिशु (shishu): This is the most formal word for "baby" and is often used in medical or scientific contexts.
Informal:
* बच्चा (bachcha): This is the most common and informal word for "baby." It can be used for both boys and girls.
* बच्ची (bachchi): This is the word for "baby girl" in Hindi.
* बेटा (beta): This is the word for "baby boy" in Hindi.
* छोटा (chhota): This means "small" and is often used to refer to a baby, especially a newborn.
* नन्हा (nanha): This also means "small" and is used to refer to a small baby.
The best word to use will depend on the specific situation.