Eutherianism refers to the classification of eutherians, which are a group of mammals known as placental mammals. This group includes a diverse range of species, such as humans, dogs, and whales, characterized by their complex placenta that allows for extended gestation periods and more developed young at birth compared to marsupials or monotremes. Eutherians are distinguished from other mammalian groups by their reproductive and developmental processes, contributing to their adaptability and evolutionary success.