The Collaborative International Dictionary of English v.0.48:

Vigilance \Vig"i*lance\, n. [L. vigilantia: cf. F. vigilance.]
   1. The quality or state of being vigilant; forbearance of
      sleep; wakefulness.
      [1913 Webster]

   2. Watchfulness in respect of danger; care; caution;
      circumspection. --Cowper.
      [1913 Webster]

            And flaming ministers to watch and tend
            Their earthly charge; of these the vigilance
            I dread.                              --Milton.
      [1913 Webster]

   3. Guard; watch. [Obs.] "In at this gate none pass the
      vigilance here placed." --Milton.
      [1913 Webster]

   Vigilance committee, a volunteer committee of citizens for
      the oversight and protection of any interest, esp. one
      organized for the summary suppression and punishment of
      crime, as when the processes of law appear inadequate.
      [1913 Webster]

WordNet (r) 3.0 (2006):

vigilance
    n 1: the process of paying close and continuous attention;
         "wakefulness, watchfulness, and bellicosity make a good
         hunter"; "vigilance is especially susceptible to fatigue"
         [syn: watchfulness, wakefulness, vigilance,
         alertness]
    2: vigilant attentiveness; "he keeps a weather eye open for
       trouble" [syn: watchfulness, vigilance, weather eye]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "vigilance":
   alertness, attentiveness, caution, circumspection, custodianship,
   custody, eagle eye, guard, guardedness, guardianship, invigilation,
   lookout, monitoring, observance, peeled eye, proctoring, prudence,
   qui vive, sharp eye, stewardship, surveillance, tout, vigil,
   wariness, watch, watch and ward, watchful eye, watchfulness,
   watching, weather eye



Bouvier's Law Dictionary, Revised 6th Ed (1856):

VIGILANCE. Proper attention in proper time.
     2. The law requires a man who has a claim to enforce it in proper time,
while the adverse party has it in his power to defend himself; and if by his
neglect to do so, he cannot afterwards establish such claim, the maxim
vigilantibus non dormientibus leges subserviunt, acquires full force in such
case. For example, a claim not sued for within the time required by the acts
of limitation, will be presumed to be paid; and the mere possession of
corporeal real property, as if in fee simple, and without admitting any
other ownership for sixty years, is a sufficient title against all the
world, and cannot be impeached by any dormant claim. See 3 Bl. Com. 196, n;
4 Co. 11 b. Vide Twenty years.