OK, according to Theravadin Buddhism, the definition of a Bodhisattva is a person, or being, who has vowed to become a Buddha. And there are certain conditions concerning their vow.
There are differences between Theravadin and Mahayana teachings on Bodhisattvas. Within Theravada, there is a very limited number of Bodhisattvas. There might only be a total of twenty-five in total existence. It's a different type of teaching in the Mahayana tradition, which I will not get into here.
According to the Theravadin tradition, the Buddha was a man named Siddhartha Gautama. Five hundred or so lifetimes prior to his enlightenment, he had a chance to become enlightened. He was wise enough. His Paramis were high enough to become enlightened, but before becoming enlightened, he became really inspired when he actually saw another Buddha, the Buddha at that time. And in his inspiration he chose to renounce his opportunity to become enlightened at that time, and he instead vowed to become a Buddha.
Now, vowing to become a Buddha does not automatically make you a Bodhisattva. According to Theravadin Buddhism, you have to be a man and make the vow in front of a living Buddha. You also must have high enough Parami development that you could get enlightened at that time and become what they call an Arahant. Not only that, you must also be living as an ascetic, in the same style as a Buddha, living in the woods and so on. Not only that, but the Buddha then must look into the future and actually see whether it's going to be true or not, and then the Buddha must state that it will come true.
There are not too many people that this happens to. That's what a Bodhisattva is, according to the Theravadin tradition. Then, for five-hundred or so lifetimes, this person works towards complete perfection of the Paramis. Not just getting the Paramis very high, but getting them all perfect, only the Buddha gets them perfect. So a Bodhisattva actually has to work lifetime, after lifetime, after lifetime, after lifetime, in order to perfect the Paramis. All ten of them - perfect. And when they are finally perfect, they become a Buddha at that time.