What do historians mean by the term “axial age”?

The term "axial age" was coined by the German philosopher Karl Jaspers.  It is used to refer to the time period that lasted from 800 BC to 200 BC.  The axial age is said to be a time when important ideas sprang up (independently) in various places in the world.  These places included India, the West, and China.

The term "axial age" is most important to the history of religion and of philosophy.  During this time, the Buddhist religion arose, as did Zoroastrianism.  Confucius lived during this time in China and Socrates lived in Greece.  So this was a time when many important religious figures all lived.