A space which is completely empty of all matter, including air.
A space exhausted of its air content.
A longstanding concept of natural philosophy that provides a useful tabla rasa on which to construct our models of reality: a completely empty space devoid of all matter, a void. In the fifth century B.C.E., Leucippus of Miletus and Democritus of Abdera maintained that the world consisted of an infinite number of tiny atoms moving randomly in an infinite void. Aristotle in the fourth century B.C.E. vigorously opposed the concept; a vacuum, total emptiness, in his physics would result in particles moving with infinite velocity, a non¬ sensical idea. But in the next century Epicurus again favored the void hypothesis. The Stoics, materialists like the Epicureans but otherwise opposed to them, allowed the existence of an infinite void outside the cosmos, that is, outside our universe. Much later, in the 13th and 14th centuries, theologians came to an uneasy compromise with Aristotle’s teachings in the course of which they also favored the concept of an infinite void, if only so that God had space to create other universes than our own if he so wished.
In the 17th century, Galileo encouraged his student Evangelista Torricelli to experiment with producing a vacuum in a vessel. Soon after Galileo’s death, Torricelli showed that the space above the mercury in a glass barometer tube containing no air was an effective vacuum. But it would be a mistake to think of a Torricellian vacuum as empty. There is mercury vapor in the space, about a million times fewer molecules than in the same volume of air. That, however, is still a lot of molecules about 500 million million million in each cubic inch.
Within the confines of an X-ray tube, there exists a region that is utterly void of air, gas, or any form of substance. This space is situated between the cathode and anode and is characterized by an absence of matter.