Set (a Card)
Set a Card
To Set is to place a Card face-down on the Field.
You can Normal Set Monster Cards from your hand onto Main Monster Zones, which places them in face-down Defense Position horizontally on the Field. You can only perform one Normal Summon or Set per turn.
Set Spell and Trap Cards
With the exception of Field Spell Cards (which you Set in the Field Zone), you Set Spell and Trap Cards onto the Spell & Trap Card Zones. Cards that are Set in the Spell & Trap Zone or Field Zone are placed vertically.
You cannot Set Pendulum Monsters in the Pendulum Zone as Spell Cards. Prior to the introduction of Extra Monster Zones, Pendulum Zones were separate Zones, so no Cards could be Set in them; after the introduction of Extra Monster Zones, two Spell & Trap Zones are used as Pendulum Zones instead, although they are not treated as Pendulum Zones if there is not a Pendulum Monster in them (such as if a Spell or Trap Card is Set in them).
Set by Card Effect
If you Flip a Card on the field face-down (for example, by the Effect of "Book of Moon" or Special Summon a Monster in face-down Defense position for example by the Effect of "The Shallow Grave") it is still considered a form of Setting.