Ultra is an adjective, any thing beyond generally defined and going further beyond is an ultra. Probably, cannot define numerically
Ultra relates something to an extreme end... It can not be related with any definite numerical value. Like Ultrasonic frequency means frequencies > 20K Hz.
Ultrapure water means water having 1-15MOhm/cm resistivity.
UV rays mean electromagnetic radiations with greater wavelength than the X-rays but shorter wavelength than the visible light.
In this way, ultrafiltration has a different definition.
Ultra does not correlate to a particular power of 10.
In addition to the answers above:
VHF (Very High Frequency) is 30..300 MHz (in German already ULTRA-Kurzwelle)
UHF (Ultra High Frequency) is 300 .. 3000 MHz.
Historically, 'ultra' was use to indicate something that went beyond what was already known but considered to be 'top'. UV as 'violetter than violet' (and invisible), ultrasound in the sense of 'beyond sound' etc.
Regards
the word ULTRA is a very vague term.it basically means going beyond the defined or going beyond the the already set limits.
moreover the word is more related to general english rather than mathematical/physical terms. In mathematics or physics we generally deal with the things,the values of which are defined ,as high as centillion(10^303) and similarly lowest number are also defined.
the word ULTRA is more famous among "not so smart" people as we see in the mobile phones ads referring the model as ultra-slim and the likes.As engineers, we are more interested in the accurate dimension of the things.
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