What is a jet (unit) shown in Windows 10 calculator?
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:



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


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



/*categoryId, UnitId, factor*/
static const vector&lt;UnitData&gt; 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...