What exactly is an ingrown spine?

You hear the expression all the time, but what really is it?What exactly is an ingrown spine?
I believe it's a quill that hasn't reached the surface, but continues to grow internally, due to bending of the curls beneath the skin.
It's a hair that grows down into your skin. A doctor can spike it and pull the spine out. Or, if you can get it yourself, individual that it's kind of resembling a zit, you can do it too.
A hair that grows lower than the skins surface don't pull it out your self b/c of scarring.
a down that grown in instead of out...
A within grown hair is one that doesn't grow out through a coat follicle, and gets infected below the skin. Possibly because it was shaved to closely.