What is the difference between a torpedo and missile?

Question

In Earth-speak, the main difference between a missile and a rocket is that the former is self-guided while the latter is generally unguided. A torpedo is considered to be an underwater missile and therefore, a purely aquatic weapon by definition.

However, in Star Wars, the terms torpedos and missiles are both used for different non-aquatic projectile weapons. We have the X-Wing's proton torpedoes, the Y-Wing's ion torpedoes, the A-Wing's concussion missiles and the Separatists' discord missiles. How are projectiles in Star Wars categorised between missiles and torpedoes?

Answer 1

Changing my comment to an answer as it pretty much answers the question.

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The definition of "torpedo"

Looking up the definition of the word "torpedo" nets the following result:

A cigar-shaped self-propelled underwater missile designed to be fired from a ship or submarine or dropped into the water from an aircraft and to explode on reaching a target

In other words, a torpedo is nothing more than a missile that's designed to travel in water, rather than through the air.

The other answer argues that it's the intended target that defines the distinction between missiles and torpedoes, but that seems wrong. This seems to be a case of correlation and mistaken causation.
In aquatic combat, there is not much to shoot at except for ships. Therefore, torpedoes can only really be fired at ships, in absence of any other meaningful target. That does not mean that the definition of a torpedo therefore requires it to be shot at a ship.

Similarly, just because all basketball players in the NBA wear shoes when playing a game, does not mean that it's a rule that players must wear shoes. They all just choose to wear shoes in absence of a good reason to not wear shoes.

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But why do they call some space missiles torpedoes?

Space-based combat is much closer to aquatic combat than it is to land combat. We speak of spaceships, not spacecars or spacevehicles or ...

This is pretty much entirely encapsulated by the Space is an Ocean trope. (TV Tropes warning!)

Maybe it's the romance, maybe it's the adventure, maybe it's the obvious parallels to the Age of Exploration, but for some reason, when people write about space, they tend to make parallels to the sea, as President Kennedy (himself a former naval officer) did in his "Space is the new ocean" speech. Often, it goes far beyond metaphor. Science Fiction writers frequently use nautical analogies for pretty much everything in space, and fill in the gaps in their own knowledge about spaceflight with details specific to sea travel.

To some extent, Space Is An Ocean is a Justified Trope: not only was space thought to be some kind of fluid until the turn of the 20th centurynote , but seafarers long ago evolved the organizational techniques necessary to safely operate a self-sufficient vessel in a potentially hostile environment for an extended period of time, and it makes more sense to adopt nautical administrative and logistic features (and the terms for them) instead of inventing everything from scratch.

I won't list all examples (that's what the link is for), but some do stand out as commonly accepted space tropes:

• Space militaries almost always use naval ranks, as opposed to army ranks or the RAF system, and soldiers stationed in space are usually called "marines", e.g. the "space marines" of Aliens, Doom, Marathon, StarCraft, etc. Starship Troopers did not call its soldiers marines though it could be argued that it established the archetype for later space marine forces. Even in real life, space explorers are called "astronauts" and "cosmonauts".
• Spacecraft even have "lifeboats"—generally called escape pods or something similar—despite the concept being largely impractical in case of realistic space travel.
• In space, hovering things have to move up and down slightly. (Note: this refers to making the scene believable for the viewer, rather than being physically sound)

It even makes more sense when you compare submarines (not just boats) to spaceships:

• Both move in three dimensional space.
• Prolonged exposure to space (or water...you get it) outside the vessel can be deadly (if the sub is currently at depth).
• Visual displays of the outside environment are less than useless (both space and the briny deep are inky black).
• The torpedo analogy works better as well.

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Conclusion

Why are space missiles called torpedoes?

Because it's a common trope to use naval terms for space-travel, due to the similarities.

So they are torpedoes. But why do they also call them missiles in Star Wars?

Going by the definition I linked above, a torpedo is an underwater missile. Therefore it would be correct to call a torpedo a missile.
All torpedoes are missiles, but not all missiles are torpedoes.

Regardless, since it's accepted to refer to space missiles as torpedoes, that inherently means that it's also correct to call them missiles.

So to answer your question directly: In regards to Star Wars and other forms of space combat, all missiles can be considered torpedoes, and all torpedoes can be considered missiles. There is no meaningful distinction between the two.

Answer 2

One important distinction is that a torpedo is generally fired by ships, at ships, while missiles may be fired by pretty much anything, at nearly any target. Occasionally, something else (aircraft, for instance, occasionally shore batteries) may launch a torpedo, but only at a ship or submarine (even though there are shore targets torpedoes could attack).

So, the main distinction in a non-aquatic setting like Star Wars is the target the weapon was designed to attack: torpedoes are dedicated anti-ship weapons, while missiles might have multiple target types.

Answer 3

The difference between rockets, missiles, bombs, and torpedos is simple. Rockets are self propelled unguided weapons. Fired from anywhere, doesn't matter, you hit fire ans they go wherever the launcher was aimer until fuel runs out (or in space where there is no air resistance u til thwy hit something). Missiles are self propelled target peojectiles that track a target or are set to hit a target they are not directly aimed at. They can change direction on their own power and can either seek out and inmact a target or be set to go somewhere. Torpedos (outside of the water) are non self propelled targeted devices that lock on a target and are accelerated through a tube or by a casing (like proton torpedos with a huge base section thay never leaves the aircraft and a small warhead with no engines that gets fired). They are set out and can use their own power to change course, but primary thrust is derived feom the initial launch force. Bombs are the rockets but not self propelled. They have a thing in sight, and then are simply released with the knowledge the momentum of the craft that launched them or the gracity in an environment will get then toward their target. Even though some are guided, they are not launched and are not self propelled. They are dropped or lightly ejected.

Answer 4

Functional difference in games

In the X-Wing and TIE Figher series of space flight simulation games, concussion missiles were the fastest and most agile warhead, while torpedoes were much slower and less maneuverable, but packed significantly bigger punch. The limited power of concussion missiles meant they were only useful against enemy starfighters, while the limited maneuverability of proton torpedoes meant they were only useful against large, slow-moving targets such as capital ships and space stations.

Why call them "torpedoes"?

"Proton torpedo" is obviously lifted from "photon torpedo", a commonly referenced warhead in the Star Trek universe.

Star Trek probably referred to them as torpedoes because of the naval connotation. The structure of Starfleet is more analogous to a navy; Starfleet's ranks are naval ranks (e.g. Commodore, Admiral) and the USS Enterprise itself is named after a naval vessel.

Answer 5

A missile is a projectile rocket propelled by combustion product reaction with a guidance package to steer and traverse to it's destination. A torpedo is a self propelled projectile with a hardened casing for proximity detonation and is powered by a engine/motor for transit. In the Trek universe; given that photon torpedo's are powered by the same reaction as a starship (matter/antimatter reaction) the weapons choice of nomenclature is similar. So a proton torpedo is likely propelled using the same means as the ship that fires it.