Set
To Set is to place a card on the field face-down.
When you Set a Monster Card, you place it on a Monster Card Zone face-down in Defense Position. So far, it can only be Set in Attack Position with the effect of "Darkness Approaches".
When you Set a Spell/Trap Card, you place it on a Spell & Trap Card Zone face-down in vertical position, except Field Spell Cards, which can only be placed on the Field Card Zone, even if it is face-down.