Roughly defined as a one foot radius around a person. It can only be entered by close friends, family members, significant others, etc. You know when you're in a person's personal space. You can sense it!
Whoa, man! Personal space! Arm's length, bucko!
An area around a person, within which other people make them nervous, except for intimacy. Also, space and my space.
You're invading my personal space.
Get out of my space!