adjective Meteorology.
(of high-altitude clouds) visible during the short night of the summer.

1885–90; nocti- + lucent

noctilucence, noun Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
noctilucent (ˌnɒktɪˈluːsənt)
shining at night, usu. of very thin high altitude clouds observable in the summer twilight sky

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
noctilucent   (nŏk'tə-l'sənt)  Pronunciation Key 
Luminous at night. The term is used especially to describe certain high atmospheric cloud formations visible during summer nights at high latitudes.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
They are among the first such noctilucent clouds pictured this year.
After asking several astronomers, they are possible noctilucent clouds, airglow
  or faint aurora.
They are called noctilucent clouds, or night-shining clouds.
For years, scientists have puzzled over the observed increase in these
  noctilucent clouds.
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