Oxen

An ox, to early American farmers who used the beast, was a mature castrated male belonging to the domestic cattle family, or genus Bos, most likely trained (like draft horses, some never got trained) to work, and at the end of its life inevitably used for meat.
A steer, by contrast, is also a castrated male of the genus Bos, but is a younger animal that may not be trained, or may not be strong and mature enough for hard work. In the United States a steer is not considered an ox until it is four years old, by which time it is considered large enough and mature enough for any work required of it.
In Australia and elsewhere, an ox is a called a "bullock." Same beast, but a different culture. New England teamsters sometimes call oxen "bulls," even though the animals have been castrated.
To be culturally and historically accurate when defining an ox, we must use the "right" definition as provided by the Random House Dictionary, which says that an ox is "The adult castrated male of the genus Bos used as a draft animal and for food."
Although, by United States standards, this definition is correct culturally, historically, and scientifically, it has its problems. Only two species in the genus Bos used for work are called "oxen"—Bos indicus (Zebu-type cattle with humps) and Bos taurus, the European breeds (no humps). Other species in the genus Bos, such as yaks, may be worked, but are not called "oxen."
To define the word "ox" as encompassing all animals in the bovine family would include a lot of species that are not even domesticated. And it would include both males and females. This might be acceptable in some broad, casual context, but not if scrutinized by ox teamsters and agricultural historians in the United States.
Most oxen weigh about the same as a mature bull of the same breed, but the ox grows taller and leaner in the neck and chest. The following breeds and crossbreeds are most commonly used as oxen in the United States today: