Baby is a general term used to refer to humans under the age of four years and can be used in reference to newborns (from birth to 28 days), infants (from birth to 1 year), and toddlers (1 to 4 years). During the first year of life, also known as infancy, a baby develops at an astounding rate, learning how to focus their vision, respond to verbal cues, sit up on their own, and so on. Throughout the toddler stage, babies become much more mobile and independent as they toddle around and begin learning how to speak and socialize.