Look up, there’s space. Astronomically speaking, it’s right there, just outside a thin layer of atmosphere. But how far away is it? How high is space? Space is defined by the point at which the Earth’s atmosphere ends, and the vacuum of space takes over.
The first official definition of space came from the National Advisory Committee for Aeronautics (the predecessor to NASA), who decided on the point where atmospheric pressure was less than one pound per square foot. This was the altitude that airplane control surfaces could no longer be used, and corresponded to roughly 50 miles, or 81 kilometers.
Any NASA test pilot or astronaut who crosses this altitude is awarded their astronaut wings.
