In the Star Trek normal universe, almost all of Starfleet's starships were named with the prefix USS: Count from list. Some popular ones: USS Enterprise - series, USS Stargazer, USS Exeter, USS Defiant, USS Excelsior, USS Voyager, USS Titan, USS Reliant, USS Equinox, USS Grissom, USS Relativity, etc.

In the mirror universe, Starfleet's starships were named with the prefix ISS. We don't have a big list, but we do see these: ISS Enterprise NX-01, ISS Enterprise NCC-1701, ISS Avenger.

What's the full form of USS & ISS in Star Trek canon?

In the real world, the US Navy uses USS in ship names, which is reasonable as "United States Ship", but why is it used in Star Trek canon (in which there's no US to control starships)?

In the mirror universe, USS is replaced with ISS. Why? Does ISS have a meaning opposite from that of USS?

  • In the UK they actually use HMS which stands for His/Her Majesty's Service, which is pretty British. The original US designations were US with the next letter being the type of ship(USF for a frigate as an example). Rosevelt had that changed to USS which stands for United States Ship.
    – user72787
    Commented Oct 12, 2016 at 18:11
  • "We don't have a big list": Since it's a mirror universe, it is presumed that all USS ships would have ISS counterparts. Therefore the list of ships in mirror universe would be exactly the same as that of those in the prime universe, except for the prefix being ISS.
    – ADTC
    Commented Apr 4, 2020 at 0:31

4 Answers 4


USS has had several meanings.

From an article on TheGeekTwins exploring the same question:

In three episodes of the original series it was called by the name "United Star Ship." In the "The Cage," the Enterprise is refered to as the "United Space Ship Enterprise." Captain Kirk also used the term in "Elaan of Troyius" and Dr. McCoy did in the episode "Space Seed."

To confuse you even more, in the UK comic series it's known as "Universe Star Ship."

"United (Federation) Star Ship" is the most common definition you often see cited through other sources, linking the designation with the United Federation of Planets, however as pointed out by one researcher, the United Federation of Planets did not exist at the time the USS Enterprise NX-01 was first commissioned.

ISS doesn't seem to have explicit, high level canon to support it's definition, however in the game Starfleet Command (and its sequels) as well as within "The Mirror Universe Saga" (DC Star Trek comic, 1984-85), ships bearing this moniker make up the Imperial fleet and therefore carries a strong indication that it means "Imperial Star Ship."

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    the United Federation of Planets did not exist at the time the USS Enterprise NX-01 was first commissioned - No, but United Earth did, just barely (the last few holdouts joined in 2150). It almost certainly still means United Star Ship, with "Earth" replacing "Federation" in parentheses.
    – Izkata
    Commented Aug 9, 2012 at 22:53
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    @Sachin: NX-01 is not "USS Enterprise". It actually doesn't have a prefix, though in pre-broadcast promotions it was advertised as the "SS Enterprise". Commented Jan 28, 2015 at 0:46

This was a practice that originated from the "United States Ship" designations such as USS Enterprise (CV-6), a carrier in the United States Navy. Star Trek was emulating and referencing the military of its time period (the 1960's) and the USS designations and the Naval Construction numbers on the hulls were thought to be a way to show how naval traditions had migrated into space.

  • The USS part of the ship designation - United Space Ship
  • They included registry numbers - Naval Construction Contract numbers (NCC-1701 for example) to designate versions and variations to add further distinctive identifications.
  • In the Mirror Universe, ISS stood for (Imperial Space Ship).

The Federation Starfleet used the "USS" prefix on the hulls of their starships and as part of a starship's title. USS was referred to as standing for either "United Space Ship" (TOS: "The Cage", "The Menagerie, Part I", "Space Seed", "The Gamesters of Triskelion", "Patterns of Force", and more) or "United Star Ship." (TOS: "The Squire of Gothos", "Court Martial") This designation was used as early as 2167 on the Daedalus-class starship, the USS Essex. (TNG: "Power Play"). -- Memory Alpha > USS designation

  • Any reason to add last paragraph in answer?
    – user931
    Commented Mar 16, 2012 at 22:39
  • 2
    I noticed that trend in Star Trek, to use "Imperial Empire." They use that in ST:TNG when talking about Klingons. I wonder if it ever occurred to them that it's redundant.
    – Tango
    Commented Mar 16, 2012 at 23:38
  • "In the mirror universe, USS is replaced with ISS. Why? Does ISS have a meaning opposite from that of USS?" This question explains the second paragraph. Commented Mar 17, 2012 at 2:35
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    I have noticed a preponderance of answers seem to get the nod as the answer of choice if the answer references a previous wiki. What exactly is the value add if all we do is list a reference to a previous wiki? I can do that with ease with 90% of the questions here. I thought we were trying to do more than repeat other wikis. If I am wrong let me know... Commented Mar 17, 2012 at 2:37
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    There is no reference to "Imperial Empire" in Mirror, Mirror, so your last paragraph has major flaws.
    – aramis
    Commented Mar 17, 2012 at 8:25

The origin of the term "U.S.S." is originally "United States Ship," a form commonly heard by persons dealing with US naval vessels in any capacity. This is one of a great many such terms used in the real world wet-navies. Even in the original proposal, just such a term was used in the "excerpted orders" - "SS Yorktown"

The following is a non-exhaustive sampling of real world ship name prefaces:

FV: Fishing Vessel, Factory Vessel
MV: Motor Vessel
RMS: Royal Mail Ship {UK}
SS: Steam Ship
RV: Research Vessel
USS: United States Ship
IJN: Imperial Japanese Naval (ship)
JDS: Japanese Defense Ship
HMS: His/Her Majesty's Ship {UK}, Hans/Hennes Majestäts Skepp {Danish}
HMAS: His/Her Majesty's Australian Ship {Australia}
USCGC: United States Coast Guard Cutter {US, rarely used except formally}
RFS: Russian Federation Ship {used for Russian vessels, but not by the Russians!}

The Writer's Guide for Star Trek gives "United Space Ship" on page 1 (3rd visible page in the PDF). It should be noted that this appears to be the guide for season 2 - it makes no reference to Checkov, but does reference season 1's success.

ISS would be "Imperial Star Ship" - imperial being the adjectival form indicating ownership by an Empire; in the case of Star Trek's Mirror Universe, the Terran Empire. While this is not specified in TOS's Mirror Mirror, Enterprise's In a Mirror Darkly, Part 1 has Hoshi saying, "Another Terran Empire? Another Starfleet?" This implies that the Empire referred to in the ISS is the Terran Empire in whose Starfleet the Mirror Hoshi speaking serves.

IKV is used for Klingon Vessels in fannon; it means "Imperial Klingon Vessel" and does not appear on screen.

IKS is used in DS9, standing for "Imperial Klingon Ship." Both are used as name prefixes in DS9's The Sons of Mogh, tho not for the same ship.

As an aside: We also know that the Terran Empire has fallen by the time of DS9 doing a mirror universe episode. DS9 and Voyager would therefore not be dealing with the Terran Empire.


USS was simply a mistake. At the time the first script came in the script writter didn't realize that us was no longer valid for the film since UFP would be more appropriate but the public was more familiar to associate uss with a vessel name than U FP so in order to keep up with the best marketing practices they (the producer's) kept USS instead of UFP.

  • 2
    Wow... That's a nice information. But, do you have any citation?
    – user931
    Commented Jul 6, 2014 at 0:38
  • 1
    What script? What first film? You really need to back up any dramatic assertions like this...
    – Valorum
    Commented Jul 6, 2014 at 0:54
  • 1
    I don't see how "UFP" would be an appropriate ship prefix. You don't use "USA" in the States, or "GB" in the UK. Furthermore, you make wild assertions with absolutely no evidence! Commented Jan 28, 2015 at 0:50

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