What exactly is an outpost?

Hi all,

I'm currently reading the first chapter of Simple Chess by Michael Stean and it talks about outposts. I've seen and heard this term over and over but I never really understood what it meant. Now I'd like to fully understand this book, so I tried to figure out what it exactly meant.

The book shows a diagram with only pawns:

It says: From this we see very strong squares or outposts for his pieces on c4 and d5. They are strong because neither can be challenged by a black pawn.

So I thought: Ok, so an outpost is a square that can never be attacked by the opponents pawns. In that case, doesn't d6 count as an outpost? Why not? (The book goes even further by saying it's 2-0 for white in outposts, so I assume it's not an outpost).

By the way, this is the diagram with pieces. Maybe the pieces have influence in whether a square is an outpost or not?

So I went to Wikipedia, because as everyone knows, if Wikipedia says something it has to be true :P

However, Wikipedia makes it even more confusing. It says:

An outpost is a square on the fifth, sixth, or seventh rank which is protected by a pawn and which cannot be attacked by an opponent's pawn.

But then it shows a diagram saying that c4 is an outpost. But that's on the fourth rank. 

And even more, in the first diagram c4 was also an outpost. But that field wasn't attacked by a pawn. 

So I'm getting lost in this. Can someone give me a clear explanation on what exactly is an outpost? 

An outpost is a square that cannot be attacked by enemy pawns, and is controlled by one of yours.(I think)

White would have an outpost for his knight on D5 in the example.

I think that a good book would call d6 in the first diagram a "square of incursion" or "entry square" - it could still be useful to invade by putting  a piece there. But an "outpost" needs the support of a pawn. One of the common tactics for an outpost is to get the opponent to take the piece that is outposted, then retake with the pawn and in doing so you open up a new line for another piece - often a rook or queen or bishop. That's another reason for using a knight as the outpost - once the opponent takes the knight you still have the heavy pieces for the lines that are opened.