Real Dictionary

baby talk
uncountable noun: Baby talk is the language used by babies when they are just learning to speak, or the way in which some adults speak when they are talking to babies.

baby talk in American English
playful, often nonsensical talk used to amuse or imitate a baby
noun: the speech of children learning to talk, marked by syntactic differences from adult speech and by phonetic modifications like lisping, lalling, omission and substitution of sounds, etc

baby talk in British English
noun: the speech of very young children learning to talk