going beyond what is usual or ordinary; excessive; extreme.
an extremist, as in politics, religion, fashion, etc.
(initial capital letter) Military. the British code name for intelligence gathered by decrypting German wireless communications enciphered on the Enigma machine during World War II.

independent use of ultra-, or shortening of words prefixed with it

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a prefix occurring originally in loanwords from Latin, with the basic meaning “on the far side of, beyond.” In relation to the base to which it is prefixed, ultra-, has the senses “located beyond, on the far side of” (ultramontane; ultraviolet ), “carrying to the furthest degree possible, on the fringe of” (ultraleft; ultramodern ), “extremely” (ultralight ); nouns to which it is added denote, in general, objects, properties, phenomena, etc., that surpass customary norms, or instruments designed to produce or deal with such things (ultramicroscope; ultrasound; ultrastructure ).

< Latin ultrā (adv. and preposition) on the far side (of), beyond, derivative of *ult(e)r- located beyond

ne plus ultra

[ne ploos ool-trah; English nee pluhs uhl-truh, ney]
the highest point; acme.
the most intense degree of a quality or state.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
ne plus ultra (ˈneɪ ˈplʊs ˈʊltrɑː)
the extreme or perfect point or state
[literally: not more beyond (that is, go no further), allegedly a warning to sailors inscribed on the Pillars of Hercules at Gibraltar]

ultra (ˈʌltrə)
1.  extreme or immoderate, esp in beliefs or opinions
2.  an extremist
[C19: from Latin: beyond, from ulter distant]

1.  beyond or surpassing a specified extent, range, or limit: ultramicroscopic
2.  extreme or extremely: ultramodern
[from Latin ultrā beyond; see ultra]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

prefix meaning "beyond" (ultraviolet) or "extremely" (ultramodern), from L. ultra- from ultra (adv. and prep.) "beyond, on the further side," from PIE *al- "beyond." In common use from early 19c., it appears to have arisen from Fr. political designations. As its own word, a noun meaning "extremist" of
various stripes, it is first recorded 1817, from Fr. ultra, shortening of ultra-royaliste "extreme royalist."

ne plus ultra
"utmost limit to which one can go," 1638, from L., lit. "no more beyond," traditionally inscribed on the Pillars of Hercules.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

ultra- pref.

  1. Beyond; on the other side of: ultraviolet.

  2. Beyond the range, scope, or limit of: ultrasonic.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Example sentences
It degrades in ultra-violet light but provides control on plant surfaces for up
  to seven days.
We sat on the roof deck, where the ambience was ultra-simple: a sheet of brown
  paper laid across a picnic table.
Previous experiments had managed the feat at ultra-cold temperatures, or by
  using exotic semiconductors such as gallium arsenide.
Also much ozone to continually block out ultra-violet rays.
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