The two vertical corridors between wing and centre. Coaches split the pitch into five such lanes — the half-space is the one nobody feels responsible for.
In training the final third is often marked out with cones into five vertical corridors: two wings, two half-spaces, one centre. The half-space matters because responsibility for it is unclear — you are too far inside for the full-back and too far outside for the centre-back. That gap is where you are hardest to pick up. The typical amateur habit is running everything down the wing, because that is where the space looks biggest. From the wing you can only play the ball on in one direction; from the half-space you have forward, inside and outside. Pack three or four players into one half-space on purpose and the opponent has to overcompensate towards it — which tears the far side open. What often gets missed: the half-space is a zone, not a position. You occupy it and vacate it depending on where the ball is, otherwise you take the space away from a team-mate.