The name of this article is unofficial, or it uses unofficial terminology. It may change at any time as official terms are found or created.
A series is a group of three or more cards that share an appearance in artwork, name, or effect similarity, but are not genuine archetypes due to lacking any support cards. Series commonly, but not exclusively, comprise themed groups used by characters in the anime or manga.
A group of cards may be a series in one media and an archetype in another. Examples of this include the "Shark" and "White" cards, which are both archetypes in the anime and manga, respectively, but lack any specific support in the TCG/OCG. Also, a member of a series may also be a part of an archetype, an example being the "Ultimate Insect" series, whose members are all also included in the "LV" archetype.
From a practical standpoint, the most notable difference between a series and an archetype is the fact that many archetypes, such as "Lightsworn", "Blackwing" and "Six Samurai", can all have entire Decks easily built around them thanks to their own effects and the support cards they possess. Meanwhile, card series do not have support cards, and building a complete Deck out of a series such as "Hex-Sealed Fusion" can be a hard task. However, this can be also be an advantage, as some series-based Decks can be combined with other Deck types more easily than archetypes can, as they do not have specific support cards that could clog up Deck space.