Exact definition of a blocked pawn What exactly is a blocked pawn? Is it a pawn with any of the opponent pieces in front or does it have to be an opponent pawn? Will the pawn be considered blocked if it can capture?

Add any other criteria that defines a blocked pawn.
A "blocked pawn" is a rather general term that describes a pawn that cannot advance. It can be blockaded by either an opponent's piece or pawn, and in certain rare cases it can be blocked by a fellow pawn (i.e. doubled pawns). However, perhaps the most common example of a blocked pawn is one that is isolated. Here is an example of a typical IQP position: (isolated queen pawn)

r4rk1/pb2bppp/1p1qpn2/3n2B1/3P4/P1NB1N2/1P3PPP/R2QR1K1 w - - 0 1


It is a well known positional idea to blockade the opponent's passed pawn. The reason why the square in front of an isolated pawn is commonly used as an outpost is because the d-pawn does not have a fellow e or c pawn to kick the knight out of the d5 square. Therefore, Black's plan is to blockade the pawn, exchange off the minor pieces, and eventually win the pawn in the endgame. The following position illustrates this plan:

3r2k1/p2q1pp1/1p5p/3rp3/3P4/P2R4/1P1Q1PPP/3R2K1 w - - 0 1


Blockading your opponent's pawn is also a common idea in positions with a backward pawn:

2rq1rk1/1p3pp1/p1npbb1p/3Np3/4P3/1BP1NP2/PP1Q2PP/3RR2K w - - 0 1


Notice how the d6 pawn is blockaded by the knight on d5. Keep in mind that in positions with backwards pawns, the pawn itself is weak and the square in front of it is also begging to be occupied. A long-term plan could be perhaps to eventually win the d6 pawn.

In such, positional maneuvers are usually rooted in scenarios where the pawn is blockaded by a piece. To answer your last question, a pawn wouldn't be considered blocked if it can capture because it will have a square to go to. A blocked pawn by definition cannot move anywhere.

If you want to read more about blocked pawns and their importance in endgames, check out the Wikipedia article involving key squares and zugzwang.