Once per turn

"Once per turn effects" are a kind of card effect found on Effect Monsters, Spell Cards and Trap Cards. There are three kind of "Once per turn effects". The first is typically referred to as "Once per turn effects". The second is typically referred to as "Only once per turn effects". The third is typically referred to as "Additional Normal Summon once per turn effects".

Once per turn effects
These kinds of effects can be seen by the fact that the effect starts with "once per turn" and possibly another Activation Requirement as well.

Example:
 * Once per turn: You can detach 1 Xyz Material from this card, then target 1 Defense Position monster on the field; change it to face-up Attack Position, and if you do, it loses 600 ATK.

Typically, these kinds of effects are either Ignition Effects or Quick Effects, but they can also be Trigger Effects. These kinds of effects can only be used once per turn as long as that specific card stays face-up on the field. For example, if Photon Papilloperative activates its effect, and then is later destroyed that turn, and Xyz Reborn is used to Special Summon it back from the Graveyard, then you can once again use the effect as it has left the field, which "resets" the once per turn condition, thereby allowing it to activate once again. This principle also applies for Spell and Trap Cards. If you control multiple of the same card, you can activate both of their effects as well.

Only once per turn effects
These kinds of effects can be seen by the fact effect has either "You can only activate 1 "card name" per turn" or "You can only activate/use this effect of "card name" once per turn." These kinds of effects also apply to any card saying "once per Duel" and acts the exact same as it would had it said "once per turn" instead.

Example 1:
 * Add 1 "Spellbook" card from your Deck to your hand, except "Spellbook of Secrets". You can only activate 1 "Spellbook of Secrets" per turn.

Example 2:
 * During either player's Battle Step, while an attack is occurring: You can banish this card from your Graveyard; negate the attack. You can only use the effect of "Bacon Saver" once per Duel.

Like with "once per turn effects", these can be Ignition Effects, Quick Effects or Trigger Effects, though unlike them, these can only be used once per turn, regardless of them being reset, or you having multiple copies. This means you cannot activate multiple Spellbook of Secrets, since it states you can only activate one of them per turn. If the activation of a card or effect that states "You can only activate this effect of "card name" once per turn" is negated, then this means you can activate it once again. However, if a card states "You can only use this effect of "card name" once per turn", then even if the negation is negated, you cannot activate it once again.

Additional Normal Summon once per turn effects
Unlike the others shown, these work a bit differently. They can compared a bit to "only once per turn" as having multiple cards will allow to Normal Summon more than one time. These can be seen by the fact that they say "(You can only gain this effect once per turn.)".

Example:
 * "During the turn this card was Normal Summoned, you can Normal Summon 1 "lswarm" monster in addition to your Normal Summon/Set. (You can only gain this effect once per turn.)"

The most important thing here is that even though you have Evilswarm Castor and Fire Formation - Tensu, you can still only perform one extra Normal Summon. This is because they both have "(You can only gain this effect once per turn.)" in the end of the effect, and of course, if you have two Evilswarm Castor, you can't Normal Summon 2 extra times. Cards that allow additional Normal Summons, such as Double Summon or Pahunder are still allowed to be played if you used those cards though.