The word "哺乳类" is formed by combining the characters 哺 (meaning to feed), 乳 (meaning milk), and 类 (meaning category). The compound 哺乳 specifically refers to breastfeeding or suckling, the defining trait of animals that produce milk for their offspring. Adding 类 categorizes this group, resulting in a term that denotes mammals, the class of animals characterized by mammary glands and nursing their young.