There is no clear boundary between pressure and vacuum and the word vacuum simply refers to part of the pressure scale. Its definition is not precise but it is commonly taken to mean pressures below, and often considerably below, atmospheric pressure. What is particularly important, however, is to appreciate that a vacuum refers to a pressure measured with respect to zero pressure (that is an absolute pressure) and not with respect to ambient pressure or some other pressure - see pressure modes. Unfortunately some gauges - for example those showing the pressure reduction (from atmospheric pressure) in engine manifolds - sometimes bear numerically reversed scales marked vacuum; they are not measuring vacuum but rather negative gauge pressure.

A diagram on another page shows an extended pressure-vacuum scale illustrating that the transition from pressure to vacuum is seamless and there is no boundary.