baby ，infant ，toddler都有表示“婴儿”的意思，还有一个newborn也是这个意思。四者是有区别的。

解释如下：

An infant (from the Latin word infans,meaning "unable to speak" or "speechless") is the veryyoung offspring of a human or other mammal. When applied to humans, the term isusually considered synonymous with baby, but the latter is commonly applied tothe young of any animal. When a human child learns to walk, the term toddlermay be used instead.

The term infant is typically applied toyoung children between the ages of 1 month and 12 months; however, definitionsvary between birth and 3 years of age. A newborn is an infant who is onlyhours, days, or up to a few weeks old. In medical contexts, newborn or neonate(from Latin, neonatus, newborn) refers to an infant in the first 28 days afterbirth; he term applies to premature infants, postmature infants, and full terminfants. Before birth, the term fetus is used.

Infant is also a legal term referring toany child under the age of legal adulthood.

总结一下：baby适用于所有动物，infant特指人类小孩，开始学走路的小孩叫toddler. infant指1-12个月的小孩，有时也指出生到3岁的小孩。newborn指几个小时或者几天或者几周大的infant。医学上认为newborn是出生28天之内的小孩。