An adult or young person who behaves in an infantile way.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "baby" in a sentence
"A simple baby seat can help the baby hold a good position while eating."
"It’s a clump of cells that are not a baby until the woman has her *magic* thoughts and it turns into a *baby*."
"Nope, in this case, the term baby brain refers to a state of mind, or lack thereof."