A phrase first put forth by Aristotle and later again by Galileo to explain why lever-operatered water pumps work.
A little background on pumps: Most hand-operated pumps work in the same well (with the exception of Artesian Wells). A pipe's end is put in a liquid. Inside the pipe is a piston that is moved up and down with a handle. When the lever/handle is pushed down a one-way valve allows the air between the piston and the liquid to escape above the piston, but when the piston is raised, the one-way valve shuts so the liquid below is drawn up instead.
As said before, Aristotle explained this by saying that Nature doesn't play the 'vacuum' game and that something must be put into the place of where the air was, which is this case would be the liquid (this was before the nature of gasses was understood).
This thinking, however, doesn't explain why a pump of this manner cannot pump water more than 34 feet. This limitation had been known since ancient times, and although Aristotle's ideas were astute, it did not explain this phenomenon. In fact, it wasn't until the 16th century that that Torricelli finally discovered that this limitation, and indeed the ability to pump water in this manner as all, is not about vacuums but about atmospheric pressure.