Wild Space is the frontier of the Galaxy, separating the known parts of the galaxy from the Unknown Regions. The difference between the former and the latter is that some of Wild Space has been explored, though not extensively, while the Unknown Regions remains mysterious.
The term "Wild Space" was first used in the Great Manifest Period when discussing the area of partially explored space to the "east" of the galactic core. This area, as it became settled, eventually became known as "the Slice." Over the years, different definitions of Wild Space evolved, dependent upon what was known or unknown to the broader galactic community at the time.