Noun
That group of vertebrates which develops in its embryonic life the envelope called the amnion. It comprises the reptiles, the birds, and the mammals.
Source: Webster's dictionaryIt is just a section of the clade Amniota : the section that is left after the Mammalia and Aves have been hived off. Source: Internet
Mammalia and Aves are, in fact, subclades within the grand clade of the Amniota. Source: Internet