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From Windows 10's calculator:

enter image description here

What is a "jet" (physical unit)?

And why does it equal 480 knots or 888,88 km/h, if a jetliner's cruising speed is around 950 km/s? Or why does it equal 0.73 M, if a regular jetliner's cruising speed is 0.85 M?

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    $\begingroup$ +1 for cursing speed stated in km/s. That's actually plausible. $\endgroup$ – Harper - Reinstate Monica Mar 1 '19 at 16:21
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    $\begingroup$ I think "950 km/s" is a typo; surely the jet isn't moving at 950 kilometers each second. $\endgroup$ – Greg Schmit Mar 1 '19 at 19:53
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    $\begingroup$ Laden or unladen? $\endgroup$ – Dennis Williamson Mar 1 '19 at 21:51
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    $\begingroup$ European or African? $\endgroup$ – IconDaemon Mar 2 '19 at 0:54
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    $\begingroup$ Thank you all for all your enlightening comments. It is really a nice thing that such a silly (after all) question made that number of people laughing and happy! :> There's one thing that I always had trouble to understand. If this question is OT (while it clearly is, since it asks about non existing unit) why does it have so many upvotes and two, quite professional answers? :> $\endgroup$ – trejder Mar 2 '19 at 14:43
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It is not a unit. It is just Microsoft trying to be funny. Or to convey an idea of the magnitude.

Thanks @Jackie for pointing out that around March 2019, Microsoft has released this calculator as open source under MIT license. It is available in this Github repository. The source code sheds light on the definition of these pseudo units:

Description in ./src/Calculator/Resources/en-GB/Resources.resw:

<data name="UnitName_JumboJet" xml:space="preserve">
   <value>jumbo jets</value>
   <comment>A jumbo jet (eg. Boeing 747), used as a 
            comparison measurement unit for length. 
            (...)
    </comment>
 </data>
...
<data name="UnitName_Jet" xml:space="preserve">
   <value>jets</value>
   <comment>A jet plane, used as a 
            comparison measurement unit for speed. 
            (...)
    </comment>
 </data>

Conversion coefficients in ./src/CalcViewModel/DataLoaders/UnitConverterDataLoader.cpp

/*categoryId, UnitId, factor*/
static const vector<UnitData> unitDataList = {
        ...
        { ViewMode::Speed, UnitConverterUnits::Speed_Turtle, 8.94 },
        { ViewMode::Speed, UnitConverterUnits::Speed_Horse, 2011.5 },
        { ViewMode::Speed, UnitConverterUnits::Speed_Jet, 24585 },
        ...
        { ViewMode::Length, UnitConverterUnits::Length_Paperclip, 0.035052 },
        { ViewMode::Length, UnitConverterUnits::Length_Hand, 0.18669 },
        { ViewMode::Length, UnitConverterUnits::Length_JumboJet, 76 },
        ...
};

Apparently these definitions assume length in meter, and speed in cm/s. Thus, in Redmond the typical cruising speed of a typical jet is 478 knots. We also learn that Clippy is 3.5 cm tall.

888.88 km/h are very, very lucky speed digits for Chinese people. Any aircraft manufacturer or airline would benefit from promoting this cruising speed...

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    $\begingroup$ It just needs Clippy™ waving itself around for complete & utter uselessness. $\endgroup$ – FreeMan Mar 1 '19 at 20:22
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    $\begingroup$ @FreeMan - It almost does - look at the Length setting - for small numbers it gives you the units in "paperclips". $\endgroup$ – Darrel Hoffman Mar 1 '19 at 20:58
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    $\begingroup$ @DarrelHoffman ugh... I was kidding!!! $\endgroup$ – FreeMan Mar 1 '19 at 21:01
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    $\begingroup$ @FreeMan: so was Microsoft. :-D $\endgroup$ – pr1268 Mar 2 '19 at 23:07
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    $\begingroup$ I will do my best to refrain from down-voting simply for the presence of Clippy. :shudder: ;) $\endgroup$ – FreeMan Mar 5 '19 at 15:41
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Why does it equal 480 knots?

480 KTAS is the typical cruising speed of wide-body jetliners. KTAS means knots true airspeed (unaffected by wind). This is different from the ground speed passengers are used to (affected by wind).

Jet (unit) is not a standard unit as @bogl pointed out.

For example, the Boeing 777 Wikipedia article lists the cruise speed as:

Cruise Mach 0.84 (482 kn; 892 km/h)

Why does it equal Mach 0.73?

The speed of sound at 20°C is 343 m/s, and 480 knots is 247 m/s, so the Mach number (a ratio) is ~0.73. The higher you go, the colder it gets, and the slower the local speed of sound becomes, that's how 480 knots can end up being Mach 0.85 for jetliners.

See also:

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    $\begingroup$ Note also that since the speed of sound depends on altitude (higher altitude = lower temperature = lower speed of sound), and the Mach number is a fraction of the "local" speed of sound, the speed corresponding to a given Mach number will differ according to altitude. Microsoft is probably giving Mach numbers in terms of the speed of sound at sea level, not in terms of the speed of sound at 10,000 m. $\endgroup$ – Michael Seifert Mar 1 '19 at 14:52
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    $\begingroup$ @MichaelSeifert I think that was just the OP who converted it to Mach number at sea level, not Microsoft. The screenshot in the question doesn't show Mach number (likely for exactly the reason that it does vary with altitude and there are enough geeky people at Microsoft to know that.) $\endgroup$ – reirab Mar 1 '19 at 19:02
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    $\begingroup$ @reirab, yes, the screenshot does include Mach number. It is the first of the small numbers. $\endgroup$ – Jan Hudec Mar 1 '19 at 20:26
  • $\begingroup$ @JanHudec Ah, yes, you're right. Somehow I missed it when I looked at it before. $\endgroup$ – reirab Mar 1 '19 at 21:23

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