Oxford Dictionary definition: Small hawk trained to hunt.
Our definition: A falcon is any one of 37 species of raptor in the genus Falco, widely distributed on all continents of the world except Antarctica.
Adult falcons have thin tapered wings, which enable them to fly at high speed and to change direction rapidly. Fledgling falcons, in their first year of flying, have longer flight feathers, which makes their configuration more like that of a general-purpose bird such as a broadwing. This makes it easier to fly while learning the exceptional skills required to be effective hunters as adults.
Peregrine falcons have been recorded diving at speeds of 200 miles per hour (320 km/h), making them the fastest-moving creatures on Earth. Other falcons include the gyrfalcon, lanner falcon, and the merlin. Some small falcons with long narrow wings are called hobbies, and some which hover while hunting are called kestrels.
As is the case with many birds of prey, falcons have exceptional powers of vision; the visual acuity of one species has been measured at 2.6 times that of a normal human.