From what I understand a wall is a solid structure.
A fence is a barrier you can see through.
From what I have seen on vids showing the border wall between Mexico and the USA you can see through.
Trump might want to rethink what he wants to call the barrier between Mexico and the USA.
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bar·ri·er
['bere?r]
NOUN

a fence or other obstacle that prevents movement or access.
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fence
[fens]
NOUN

a barrier, railing, or other upright structure, typically of wood or wire, enclosing an area of ground to mark a boundary, control access, or prevent escape.
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wall
[wôl]
NOUN

a continuous vertical brick or stone structure that encloses or divides an area of land.