Contest Winner Cameo: Quite a few cards are designed by fans and they appear for a few episodes. Performapal Trump Witch appears to be the only lucky card that appears regularly.
Defictionalization: This began happening with significant frequency during the ARC-V era to cash in on nostalgia.
Before its official release in the card game, there was another version of the Seal of Orichalcos but there were only 15 in the entire world, used in a single special tournament by Upper Deck employees where players could duel them. And yes, their version was just as overpowered as it was in the show, which is why the legal version got nerfed.
Along with the (nerfed) Seal of Orichalcos, the Legendary Dragons were released but as regular Spell Cards.
Executive Meddling: The reason why Konami can't hold prize money for their tournaments is because of this trope. Kazuki Takahashi, creator and rights holder to the franchise, forbid Konami from holding prizes for their tournament since he wanted the game to be for kids, and that holding prize money for it would "be too much like gambling" in his opinion.
Fandom Nod: The "Twilightsworn" "Lightsworn" sub-archetype introduced in Code of Duelist is a nod towards the popular "Twilight" deck, which uses Dark monsters that have effects in the graveyard in combination with the Lightsworn's milling ability to summon monsters such as Black Luster Soldier - Envoy of the Beginning and mass summon Judgment Dragon.
Chaos Sorcerer was nicknamed "the poor man's Chaos" due to its Common rarity, as opposed being Secret Rare like the Envoys.
"Boss Monster" is often used to refer to any particularly powerful monster that's tied to an archetype and has a powerful effect that makes dealing with it difficult, as well as being difficult to summon and often requiring cards from said archetype to even get it on the field. Naturally, true to the name, once it's on the field, it feels like a Boss Battle trying to get it off the field.
"Solitaire decks", for decks that focus on winning turn one and not even letting the opponent get a chance to play. The name comes from the fact that, since the deck is so focused on winning turn one and not letting your opponent get a chance fight back or even play a single card that the person using it might as well be playing Solitaire instead of Yu-Gi-Oh.
"Beatstick" is often use to describe monsters that have a high ATK stat, are easily summoned, but don't have any outstanding effects. (or no effect at all, in the case of normal monsters)
"Floater" for any card that replaces itself in card advantage, either on summon or when it leaves the field. Examples include the Gadgets which keep your hand fueled with a stream of monsters as they search each other out, or the Hands which bring out the other when destroyed.
"Trish" for Trishula, Dragon of the Ice Barrier. Partly to keep its name short, mostly because people have found it fun to treat it as a female.
When decks are mixed together, they often take the suffix from one group. All Shaddoll variants end with 'dolls,' (ie, Shaddolls + Plants become Plantdolls, Trains + Shaddolls become Traindolls), Lighsworn variants end with 'sworn,' etc.
Performages and Performapals are an exception to this; they're called Performages & Pals. Also called EM-Em among Japanese circlesnote This is because Performapals are called Entermates in Japan (abbreviated as EM) and Performages are Entermage (Em), and Pepe among English-speakers. The latter has become the most popular.
Komoney has been picking up steam after the ban list where Konami not so subtly killed off all the major competitive archetypes of the early ARC-V era. It's now used frequently whenever Konami does something that blatantly "encourages" people to buy new archetypes, and only got worse after their gaming controversy.
The particular style of outfit Dark Magician wears has inspired an entire archetype of Magician cards, and of course he has many evolved forms.
In a roundabout way, it's now tradition for the protagonist's key monster to have 2500 Attack, while to parallel the Blue-Eyes, their rival's key monster has 3000 Attack.
Dark Magician Girl. Let's see: Magician's Valkyria, Gagaga Girl, Performapal Trump Witch, and Card Ejector.
The mighty Blue-Eyes White Dragon: Cyber Dragon, White Night Dragon, Red Dragon Archfiend, and especially Galaxy-Eyes Photon Dragon, most of which are played by major rivals and have equivalent attack and/or triple-fusion or Xyz variants. See below.
And for Blue-Eyes Ultimate Dragon, we have Cyber End Dragon, and Neo Galaxy-Eyes Photon Dragon. (Though the latter differs in that it is a triple Level 8 material Xyz monster.)
One can think of Red Nova Dragon as a Expy to BEUD as well considering that just to summon Red Nova takes two Tuners, meaning he usually has 4500 or more attack.
Franchise Killer: Allure Queen and Dark Lucius are believed to be the cards that finally murdered the LV archetype - Allure Queen for being too underwhelming in even its LV7 form, Dark Lucius for having its LV8 form be the only worthwhile form, and both for completely losing their effects if not Summoned using their lowest form, making Summoning either a slow (and often Unwinnable) grind.
Old Shame: The Senet cards from Cyberdark Impact. They shared a common theme that they could do things depending on which cards are in their "column" of the dueling field — which is to say, your S/T zone behind them, and the opponent's Monster and S/T zone in front of them. Even when you could get their effects off, they weren't any good, and were a very gimmicky archetype that also resulted in the creation of a new, obscure rule, namely that cards cannot be moved from the position they were played at on the game mat. Konami later admitted that the mechanic was a disaster because no one cares about card position or moving cards around, especially for convenience with the latter. That said, the new rules introduced with Link Monsters are bringing back position-dependent game mechanics, so Konami seems to have gotten over it, probably because now card position will always matter, instead of never mattering except for when you're using a small handful of cards.
Limited Special Collector's Ultimate Edition: Quite many cards are this, which kind of diminishes its value a bit; but there some among these that really are worthy of its title, examples are cards that are actually sold separately in one big deluxe package: Shinato, King of a Higher Plane was this at first. Other examples are Special Summons hyped in the Animated Adaptation and The Movie, expect to only get them in special events or packed with another product of the franchise (usually a Video Game).
No Export for You: The Anniversary Pack artwork for Dark Magician Girl was not included in the TCG release. According to most sources, this is because Kazuki Takahashi refused to edit the art for release outside Japan.
A common mistake by fans who grew up with the anime series is believing that, in the real-life card game, monsters that were Special or Fusion Summoned cannot attack on the turn they were summoned. This is false, and only happened in the anime as a Drama-Preserving Handicap, as most monsters summoned this way in the show are incredibly powerful; it would be hard to write plot if the protagonist or antagonist just drops in one or two turns.
There are a naive few who believe that cards showed in the show are the same as they are in real life. This is actually rarely the case. Usually, most cards that do pop up in the show are often rebalanced when introduced to the game, either through buffing it or nerfing it depending on the effect. In the cases where they are kept remotely similar with a few minor differences, it often causes a lot of disputes when it comes to the exact rulings of how the effects work and interact.
2ch users created a hoax so convincing, it tricked Japanese Yu-Gi-Oh! fan sites. The hoax basically stated that Lightsworn were dolls who were slaves of an evil force. This describes the Shadolls perfectly.
Prior to the official release of Primal Origin, there were sources depicting Starduston as a fake Duston version of Stardust Dragon. It also had a effect highly reminiscent of Stardust Dragon: During either player's turn, when a card or effect is activated that would destroy a card(s) on the field: You can Tribute this card; negate the activation, and if you do, destroy it. You can only control 1 face-up Starduston.
Konami got Kazuki Takahashi to do some artwork for several of the most iconic cards in the game. This included a very well drawn Dark Magician Girl card. However, when they requested that he censor it for an American release, he refused. As such, the card was never released outside Japan.
According to the Duel Art artbook by Kazuki Takahashi, Magi Magi ☆ Magician Gal was originally intended to be simply another "Dark Magician Girl" art to act as a female counterpart to the anniversary art of "Dark Magician." It was later made into a new Xyz monster.