keenly watchful to detect danger; wary: a vigilant sentry.
ever awake and alert; sleeplessly watchful.

1470–80; < Latin vigilant- (stem of vigilāns), present participle of vigilāre to be watchful. See vigil, -ant

vigilantly, adverb
vigilantness, noun
hypervigilant, adjective
hypervigilantly, adverb
hypervigilantness, noun
nonvigilant, adjective
nonvigilantly, adverb
nonvigilantness, noun
previgilant, adjective
previgilantly, adverb
supervigilant, adjective
supervigilantly, adverb
unvigilant, adjective
unvigilantly, adverb

vigilant, vigilante.

2. wide-awake, sleepless. See alert.

1. careless. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
vigilant (ˈvɪdʒɪlənt)
keenly alert to or heedful of trouble or danger, as while others are sleeping or unsuspicious
[C15: from Latin vigilāns keeping awake, from vigilāre to be watchful; see vigil]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

late 15c., from L. vigilantia "wakefulness," from vigilia (see vigil). Related: Vigilantly.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Those charged with protecting those ecosystems have to be that much more vigilant.
He is energetic and thoughtful in his research and vigilant against what should be called cable-channel alarmism.
The best thing you can do is to be vigilant and to keep pressure on the
  government to uphold rigorous security standards.
We need to be more vigilant concerning about our environment, let keep it clean
  and let reduce our emission.
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