infant
A very young baby who cannot walk or talk yet.
An infant is a very young child, specifically a baby in the first year or two of life. Infants are babies who can't yet walk or talk and who need constant care and attention. A newborn is an infant, and so is a crawling eight-month-old.
You'll also hear infant used more broadly to describe something brand new. A scientist might call a revolutionary technology still in its infancy, meaning it's just beginning to develop. A historian might describe an infant nation that recently gained independence. In these cases, the word suggests something young, undeveloped, and full of potential for growth.
All complicated, impressive things start small and helpless. Every brilliant scientist, every Olympic athlete, every skilled musician was once an infant who couldn't even hold up their own head.