I feel this 'fix' terminology may have come as a direct translation from Russian chess literature (so I see it used in Dvoretsky's books); is 'fixed' really a proper English term to describe pawns that are blockaded (especially in the endgame) by opponent's pawns and have little or no chance of advancing?
Often pawns are "fixed", it's more than them just being blockaded. For example assume Black's pawns are on b7 and a6. If White pushes his a-pawn to a5, he is fixing the b-pawn on the b7 square, since it's backwards and can't move. However, in this example no one would really think White's also "fixing" the a6 pawn, even though it's blockaded.
But then you have another case like the following: suppose Black has an isolated pawn on c5. If White pushes his c-pawn to c4, many would say he is fixing the weak c5 pawn, so that it can be easily attacked.
But assume Black had two extra pawns on d6 and e7, forming a pawn chain from e7 to c5. If White played c4, I doubt there would be many that considered White was "fixing" the c5 pawn.
So, when someone says "fix" they are most often talking about when you keep one of your opponent's weak pawns from moving. This is just arbitrary terminology though, and I don't think saying a blockaded strong pawn is "fixed" is wrong.
As far as i know, the term 'fixed pawn' is used exactly when a pawn is being blocked by other pawn, and thus can't move. The term 'blockade' is usually used when a piece is blocking the pawn, and we do use different terms for blocking with pawns and pieces because a fixed pawn (block with pawn) is static, while a blockaded pawn (block with piece) is dynamic (the blockading piece can always be kicked away from it's position, and the pawn becomes mobile again)