An amphibian, about which more below.
Also, a derogatory term for a French person.1
In addition, a slight hoarseness in the throat caused by mucus on the vocal cords (as in "a frog in the throat").
As well, a small metal holder arrayed with sharp points, placed in the bottom of a vase to hold flower stems in place.
Finally, a recessed panel in the large face of a brick.
But back to the amphibious frog. The adults are tailless, neckless, and stout-bodied with long muscular hind legs for jumping and webbed feet for swimming. They have no ears, instead sporting exposed eardrums on the sides of their heads; they have big bulging eyes. They have lungs but don't use them much; when resting in dry places they mostly breathe through their mouth lining, and when in wet places absorb oxygen through their skins. They are carnivorous and have long sticky tongues which they shoot out to catch insects and worms; some large frogs eat snakes and small mammals. Frogs have voice boxes and each species has one or more characteristic calls. At least one type plays the banjo and sings.
Frogs are from the order Anura, which includes both frogs and toads. The difference is not clear-cut: colloquially frog is used to refer to those types that have smoother, moister skin and live in damp or semiaquatic habitats, while toad refers to those that are warty and drier-skinned and prefer more terrestrial environs. Toad also seems to be applied to those frogs that secrete poison from skin glands, though biologists generally use the term toad more strictly to refer only to members of the Bufonidae genus.
Frogs live in every continent except Antarctica. Most hibernate in underwater mud and lay thousands of eggs in the early spring; the eggs are contained in a gelatinous covering that causes them to float, and are fertilized externally after they are laid. What hatches is a tadpole, a small limbless tailed larva that metamorphizes into a wee frog by the end of the summer. Some species, though, lay eggs on land that hatch into tiny frogs, with no intermediate tadpole stage. It takes several years for frogs to grow to their full size.
There are dozens of frog families, including the "true frog" (Ranidae), which are common in North America and have the misfortune of being important laboratory animals; the small tree frog (Hylidae), which sport an adhesive disk on the tips of their toes to help them cling to their arboreal environment; and the booming-voiced bullfrog (Rana catesbeiana), the largest of the North American frogs (4-8"/10-20 cm long body with legs up to 10"/25 cm long) whose legs are extensively marketed in the States, so it's not only the French who eat this!
Amphibians seem particularly vulnerable to habitat destruction and pollutants, and frogs are no exception; they suffer, as the good Oolong put it, "outright death" as well as "strange and frankly alarming mutations", of which "extra limbs seem to be the most common." Biologists have been noticing population declines in frogs in the last few years which cause many to worry that precious ecosystems are being irrevocably destroyed.
1I wondered about the origin of this slur, and thought it have derive from the French delicacy, frog legs, at which many English-speakers look askance. But it turns out there are many other possibilities: the fleur de lis is thought to resemble a frog, and may actually once have represented a frog. The French during World War II were said to resemble frogs when fully camouflaged and in hiding. The early French king Clovis had a frog as his emblem. The French used to refer to Parisians disparagingly as "grenouilles" (frogs) because the city was swampy, and the term gradually expanded to refer to all French people. Queen Elizabeth I used to affectionately call her French ambassador lover "frog". Pushkin thought "Quoi? Quoi?" ("What? What?") sounded like a frog's croak. All plausible sounding to me.