Why are the leopard and the cougar and the jaguar considered panthers?

Wild black panthers in Latin America are black jaguars (Panthera onca)

in Asia and Africa black leopards (Panthera pardus), and in North America may be black jaguars or possibly black cougars Black panthers have sometimes been regarded as forming different species from their normally-colored relatives

The name "panther" is often limited to the black variants of the species, but is also used to refer to those which are normally-colored for the species (tawny or spotted), or to white color variants: white panthers.

this information was taken from http://en.wikipedia.org/wiki/Black_panther