Mammals include the largest animals on the planet, the rorquals and other large whales, as well as some of the most intelligent, such as elephants, primates, including humans, and cetaceans. The basic body type is a four-legged land-borne animal, but some mammals are adapted for life at sea, in the air, in trees, or on two legs. The largest group of mammals, the placentals, have a placenta, which enables feeding the fetus during gestation. Mammals range in size from the 30–40 mm (1.2–1.6 in) bumblebee bat to the 33-meter (108 ft) blue whale. 

The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). All female mammals nurse their young with milk, which is secreted from special glands, the mammary glands. According to Mammal Species of the World, 5,416 species were known in 2006. These were grouped in 1,229 genera, 153 families and 29 orders. In 2008 the IUCN completed a five-year, 1,700-scientist Global Mammal Assessment for its IUCN Red List, which counted 5,488 accepted species.
according to who?
According to Mammal Species of the World, 5,416 species were known in 2006.