Bats are mammals of the order Chiroptera; with their forelimbs adapted as wings, they are the only mammals naturally capable of true and sustained flight. Bats are more manoeuvrable than birds, flying with their very long spread-out digits covered with a thin membrane or patagium. The smallest bat, and arguably the smallest extant mammal, is Kitti's hog-nosed bat, which is 29–34 mm (1.14–1.34 in) in length, 15 cm (5.91 in) across the wings and 2–2.6 g (0.07–0.09 oz) in mass. The largest bats are the flying foxes and the giant golden-crowned flying fox, Acerodon jubatus, which can weigh 1.6 kg (4 lb) and have a wingspan of 1.7 m (5 ft 7 in).
The second largest order of mammals, bats comprise about 20% of all classified mammal species worldwide, with over 1,200 species. These were traditionally divided into two suborders: the largely fruit-eating megabats, and the echolocating microbats. But more recent evidence has supported dividing the order into Yinpterochiroptera and Yangochiroptera, with megabats as members of the former along with several species of microbats. Many bats are insectivores, and most of the rest are frugivores (fruit-eaters). A few species feed on animals other than insects; for example, the vampire bats feed on blood. Most bats are nocturnal, and many roost in caves or other refuges; it is uncertain whether bats have these behaviours to escape predators. Bats are present throughout the world, with the exception of extremely cold regions. They are important in their ecosystems for pollinating flowers and dispersing seeds; many tropical plants depend entirely on bats for these services.
Bats provide humans with some benefits, at the cost of some threats. Bat dung has been mined as guano from caves and used as fertiliser. Bats consume insect pests, reducing the need for pesticides. They are sometimes numerous enough to serve as tourist attractions, and are used as food across Asia and the Pacific Rim. They are natural reservoirs of many pathogens, such as rabies; and since they are highly mobile, social, and long-lived, they can readily spread disease. In many cultures, bats are popularly associated with darkness, malevolence, witchcraft, vampires, and death.
An older English name for bats is flittermouse, which matches their name in other Germanic languages (for example German Fledermaus and Swedish fladdermus), related to the fluttering of wings. Middle English had bakke, most likely cognate with Old Swedish natbakka ("night-bat"), which may have undergone a shift from -k- to -t- (to Modern English bat) influenced by Latin blatta, "moth, nocturnal insect". The word "bat" was probably first used in the early 1570s. The name "Chiroptera" derives from Ancient Greek: χείρ – cheir, "hand" and πτερόν – pteron, "wing".