The Original Series

You may be looking for Star Trek: The Original Series.

When starting a new show, TV producers rarely can anticipate how successful it will be with the audience. But let's assume the show got popular, The Firefly Effect didn't hit it, and a row of sequels is inbound. Now, some producers play it simple and stick with season numbering. But the more creative types go for Oddly Named Sequels and thus, confront the fans with a difficult question: how do you refer to the first show, if its title is already used to refer to the entire franchise? Solution: "The Original Series".

A subtrope of Retronym.

Anime

 * Mobile Suit Gundam, though more often referred to as Gundam 0079 or sometimes First Gundam.
 * Magical Girl Lyrical Nanoha.
 * The Pokémon anime's original series mostly refers to the Kanto arc (and occasionally Orange Islands and Johto), though in an interesting case, the following series (Pokémon: Advanced Generation) was a direct continuation that followed immediately.
 * While the following theme are referred to by name, the first one is simply called "The Pokemon Theme."
 * Used straight, then averted, then used straight again by Pretty Cure.
 * The first installment is Futari wa Pretty Cure, and the Oddly-Named Sequel 2: Electric Boogaloo was Futari wa Pretty Cure plus a suffix... and so was the first cast reboot. However, later seasons dropped the "Futari wa", making the franchise Pretty Cure and freeing the first season from having to be called The Original Series.
 * However, the English, German, Italian and Spanish dubs of Futari wa Pretty Cure are simply called Pretty Cure, which makes it The Original Series all over again.
 * Tenchi Muyo! Ryo-Ohki, the OAV episodes.

Film

 * The first Star Wars movie (i.e., Episode IV) was originally titled simply "Star Wars." The title "A New Hope" wasn't added until 1981. It's caught on among the fanbase by now, but didn't catch on immediately.

Literature

 * The original novel about Artemis Fowl was titled simply Artemis Fowl, whereas the rest had Mad Lib Thriller Subtitles.
 * The first Warrior Cats series was simply called Warriors (or Warrior Cats, depending on where you live). Fans now refer to it as "the original series" or "the first arc", while all the other series and spinoffs have subtitles such as "The New Prophecy" and "Omen of the Stars".

Music

 * The Protomen's first album is simply called The Protomen. After Act II: The Father of Death came out, it retroactively became Act I: Hope Rides Alone.

Live Action TV

 * Star Trek: The Original Series is probably the most famous example. In fact this very wikiword redirected to it before this page came about.
 * Doctor Who's classic series refers to the original run from 1963-1989 (and sometimes its TV movie in 1996), to differentiate it from the revival and continuation from 2005.
 * The Kamen Rider Series has not one, but two shows simply titled Kamen Rider. In an inversion, only the original series has its name untouched.
 * The second show with the name, originally intended as a Continuity Reboot but later brought into the existing universe (during the show's run, as part of a Retool), is officially called Kamen Rider (New) in Japan and known as New Kamen Rider among English-speaking fans, though The Other Wiki in both languages calls it Kamen Rider (Skyrider) after a name given to the protagonist during the Retool.
 * In the media, the first Law and Order is commonly called the original or the flagship series.

Video Games

 * In Kingdom Hearts, the original game is usually referred to as Kingdom Hearts 1 or KH1 by the fans.
 * Something like this happened to The Legend of Zelda series- the first game was just The Legend of Zelda, and the second was Zelda II: The Adventure of Link, but every game after that was just called The Legend of Zelda: (something). The first game, therefore, can be referred to as "Zelda 1", "the original Zelda", etc.
 * The first game originally had the subtitle "The Hyrule Fantasy", but this never made it out of Japan. The corresponding text on the title screen was turned into "The Legend of".
 * Happens often where the title of the first is homonymous to the series: Metroid, Castlevania, Final Fantasy...
 * A weird case is the Metal Gear series: since the original game is called Metal Gear, the second Metal Gear 2... but the third is Metal Gear Solid, and the series is more referred by that name since practically all later games followed that name (MGS 2\3\4, MGS: Portable Ops, MGS: The Twin Snakes).
 * Since Mega Man received lots of spin-offs, the original series are referred to as "Mega Man Classic".
 * The Core Design-produced Tomb Raider games are usually referred to the "Classic" series. Some don't count Angel of Darkness as one of these, and die-hard purists only count the original trilogy.
 * After Unreal Tournament 2003 (UT2k3) and Unreal Tournament 2004 (UT2k4) came out, the original Unreal Tournament was called UT99 for its release year.
 * Each expansion for World of Warcraft has its own name: "World of Warcraft: The Burning Crusade", "World of Warcraft: Wrath of the Lich King", and "World of Warcraft: Cataclysm." Which leaves players with a conundrum: If someone asks "In which expansion did they add thus-and-such class/race/game-mechanic?", how do you answer when it was added back during the original game run, before the first expansion? Blizzard Entertainment sometimes refers to this time period as "World of Warcraft Classic", but the de facto consensus among the players is to refer to it as "Vanilla World of Warcraft".

Video Game Systems

 * The original Play Station officially became the "PS One" and its games "PS One games" some time after the Playstation2 came out. Previously, it was colloquially called the "PSX", after the abbreviation for it's developmental code name, "Playstation-Experimental".
 * While game series typically get a 1 tacked on for the first one as the second emerges, it's more complicated with Expansion Packs: the expansion is still part of the first game. Another convention to the rescue: base games that have expansions but don't have a subtitle of their own are referred to as vanilla to differentiate from the flavored expansions.

Western Animation

 * Scooby-Doo, Where Are You! (the first two seasons) are the original series. The 1978 episodes, which are part of the "The Scooby-Doo Show", are mistakenly labeled as part of the "Where Are You!" series for unknown reasons.
 * Transformers, referred to as "Generation 1" by most fans (after the Oddly-Named Sequel 2: Electric Boogaloo Transformers Generation 2 came out). Hasbro has since started using it themselves.