Baby (language)

"Baby" was the name the Eleventh Doctor gave to the language spoken by human babies. He was known to have spoken it to the infants Melody Pond and Alfie Owens. It appeared to be a one-way language, spoken by babies and heard by the recipients. The recipient — in these cases, the Doctor — then responded in standard English, which the babies seemed to be able to understand.

Judging solely by the Doctor's responses, "baby" was a complex language that had complete grammar and a wide vocabulary that included many colloquialisms. (DW: A Good Man Goes to War, Closing Time)