In Sci-Fi, particularly the softer kind, writers get a bit excited about the idea of galaxies with their big spiral arms and impressive names. So it's very common for writers to use the term "Galaxy" incorrectly to refer to things that only have one (or a handful) of stars and several planets or that are not particularly distant. In these cases all planets are shown to be roughly equally distant regardless of if they are in the same star system, a different star system or another "Galaxy".
This is probably due to the The Law of Conservation of Detail. The writer wanted to deal with a galactic scale, but have only have the resources to detail a handful of planets (which in turn can only be represented by a handful of characters in a room) and so scales back the scope of events.
Or equally it could be a case of Critical Research Failure. Both because Sci-Fi Writers Have No Sense of Scale, and because many people are scientifically illiterate, a large percentage have managed to so fundamentally confuse things that terms like "solar system", "galaxy", and "universe" are often treated as if they were the same thing.
This is particularly obvious when a character points out their "Home Galaxy" in the night sky (only the Milky Way, the Magellanic Clouds, and the Andromeda Galaxy are easily visible to the naked eye).
- In Dragon Ball Z, the seventh universe appears to be set up with four galaxies at the cardinal points of the cosmos, each overseen by a divine figure (Kaiō or Lord of Worlds), who is overseen by an even higher deity (Kaiōshin or God of the Lord of Worlds). This is, suffice to say, much smaller than the actual universe, particularly as it appears planets such as Namek are outside the jurisdiction of the North Kaiō, meaning that the heroes travel to a completely different galaxy in the span of just a few months (or even days, in Gokus case). In one the non-canonical movies during the series original run, Broly is even said to have annihilated entire galaxies, which is quite the feat for a single, human-sized individual (even one of Saiyan stock).
- In Space Battleship Yamato (also known as Star Blazers), the final major battle before reaching Gamilas/Iscandar is said to occur in what the characters call "The Rainbow Galaxy". However, onscreen, this looked more like a group of planetary masses than an actual galaxy. Presumably, the name comes from the different colour of the different planets. This designation was only used in the English language Star Blazers. In the original Japanese Yamato, it was properly called the Rainbow System.
- ROM: Space Knight's home planet Galador is (well, was until Galactus moved it) in a place called the Golden Galaxy, which never seemed to have much of note in it except Galador itself. It is apparently not very far from the Dire Wraiths' home the Dark Nebula, which is itself identified as a "galaxy," despite only being a nebula with a single star in it.
- Battle Beyond the Stars. Gelt says that the rest of the galaxy formed a protective league, who raised an army and cleaned out the Wretched Hive whose ruins he's living in. There's no mention of why this galaxy-wide league isn't doing something about Sador conquering and destroying planets, if just a single planet made them so nervous. Later as he's dying he bemoans the irony of dying on "a minor planet in a third-rate galaxy."
- Dracula 3000 is set in a region of space inconsistently referred to as the Carpathian System and the Carpathian Galaxy.
- In the original Stargate where gate coordinates are defined by constellations (i.e. patterns of individual stars) and yet the world of Abydos lies in the Kaliem galaxy. This was later retconned by the TV series to be within the Milky Way where the series handled the distances involved much better.
- Star Trek (2009) has Spock describe a supernova as threatening the entire galaxy. This is obviously not how it works, given how supernovae have happened before and we're still here. Star Trek: Picard makes it clear that only the local solar system was threatened.
- In Ann Aguirre's Sirantha Jax series, "galaxy" and "solar system" seem to be used interchangeably. A lot. The most egregious example is when the new fleet deploys just one starship to patrol each galaxy.
- The Face in the Frost features a model universe created by one of the worlds wizards, in which a whole galaxy goes supernova. A supernova is actually an exploding star.
- In Battlestar Galactica, the crew of the Galactica often use the term "galaxy" when they should have used "solar system". For example, Commander Adama says that Earth is located in "a galaxy much like our own" ...and in the last episode, the basestar is apparently the only one in the galaxy in which the Galactica is located, and the rest of the Cylon fleet is spread throughout the universe looking for the Galactica's fleet.
- Blake's 7:
- In "Duel", Travis explains that the other Federation patrols have pushed the Liberator into this galaxy. One assumes he meant solar system.
- In "Star One", a space minefield blocks the invasion route between our galaxy and Andromeda. Minefields are only effective when they guard chokepoints. In this case the invasion fleet could simply maneuver around it.
- Doctor Who: The series, especially the classic series, tends to throw around "galaxy" very casually. Early episodes use it interchangeably with "universe" and "solar system". For example:
- Laws and political organisations are sometimes described as spanning through multiple galaxies at various different points (e.g. The Dominators are said to rule the ten galaxies), whilst the humans who have settled these galaxies are usually shown flying basic rocket-ships. The Doctor gets a pass at visiting all of these places, but they are rarely shown as having any more depth than the 3-5 people representing those cultures.
- "The Daleks' Master Plan" is set at an Intergalactic Conference, with the Outer Galaxies allied with the Daleks.
- In "The Monster of Peladon" The Federation is at war with Galaxy 5, although the Expanded Universe later retcons this as a terrorist organisation with a grandiose name).
- In the "Nightmare in Silver" it is said that to stop the Cybermen, they had to blow up the Tiberian galaxy to stop them.
- The Opening Narration of The Invaders (1967) states that the titular beings are aliens from a dying planet in another galaxy. They are coming to Earth. They intend to make it their world.
- Power Rangers as a whole is guilty of this, having Rangers and villains travel to other galaxies like it's a hop, skip, and a jump away. The United Alliance of Evil is in control of multiple galaxies, and in Countdown to Destruction, launches an attack on the whole universe.
- The Twilight Zone (1959) episodes:
- "Probe 7 Over And Out". Cook refers to his and Norda's solar systems as "galaxies", even though they're only a few light years apart.
- "To Serve Man". In the opening narration, Rod Serling says that the Kanamits come from another galaxy. Since it's later stated in the episode that the Kanamit home planet is 100 billion miles away from Earth, it's clear that he should have said "solar system".
- Steven Universe - In "Log Date 7 15 2", Garnet mentioned that the Gem Homeworld is not located in the Milky Way Galaxy, where Earth resides, but in a separate galaxy still visible from Earth. This is despite the Gems being artificial life forms that don't require very specific atmospheres, but only having conquered about 58 planets (including Earth but not counting Homeworld) by the time of the Rose Quartz rebellion, whilst the average galaxy is likely to have billions of planets.
- Voltron: Legendary Defender: Despite claiming to be an inter-galactic conflict, there are never more than a handful of single-planet civilisations involved and travel between inhabited worlds treats them as being functionally next to each other.
- Silverhawks is repeatedly mentioned to take place in "the Limbo Galaxy", even if all of the events of the series happen within a single solar system and surrounding light-year-wide perimeter.
- The Great Debate in Astronomy in 1920 focused on the observation of what we now know to be galaxies, but which were originally believed to be spiral-shaped nebulae within the Milky Way.