low-birth-weight infant one that weighs less than 2500 grams at birth. This standard is routinely used for infants in developed countries, but infants born in other countries typically weigh less at birth. An infant (from Latin īnfāns 'baby, child', literally 'unspeaking') [1] is a very young human being. Baby is an alternative term for infant. The terms may also be used to refer to juvenile beings of other organisms. Infant cognitive development occurs in the Sensorimotor stage which starts at birth and extends until the infant is about 2 years of age. The sensorimotor stage is made up of six sub-stages. Infants primarily interact with the world through reflexes, such as sucking, grasping, and looking. Separated from his mother when he was just an infant, the boy grew up in a children's home. She isn't able to look after herself at all - she's as helpless as an infant. The meaning of INFANT is a child in the first period of life. How to use infant in a sentence. Infancy, among humans, the period of life between birth and the acquisition of language approximately one to two years later. The average newborn infant weighs 3.4 kg (7.5 pounds) and is about 51 cm (20 inches) long. Learn more about infancy in this article. Separated from his mother when he was just an infant, the boy grew up in a children's home. She isn't able to look after herself at all - she's as helpless as an infant. Merriam-Webster defines an infant as a child in the first period of life but doesn't give any age specifics. It describes a baby as an extremely young child. Different words are used for a baby at different stages of growth. These words are not used in the same way by everyone. For example, the word infant may be used until a baby can walk, or only until the baby is one year old. A baby from birth to about 3 months old is sometimes called a newborn. As a parent you give your children a good start in life—you nurture, protect, and guide them. Learn about developmental milestones, including emotional and social development, for babies from birth to 1 year of age. There are many things you can do to help your baby stay safe and healthy.