Set
To Set is to place a card on the field face-down.
When you Set a Monster Card on a Monster Card Zone, you must Set it in Defense Position.[note]
When you Set a Spell/Trap Card, you Set it on a Spell & Trap Card Zone in vertical position, except Field Spell Cards, which can only be Set on the Field Card Zone, even if it is face-down.
Setting a face-up card is a form of fliping it.
Notes
^ You can Set monster in Attack Position with the effect of "Darkness Approaches".