The Collaborative International Dictionary of English v.0.48:

Alertness \A*lert"ness\, n.
   The quality of being alert or on the alert; briskness;
   nimbleness; activity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

alertness
    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: a state of readiness to respond; "alerting was indicated by
       the desynchronization of the EEG" [syn: alertness,
       alerting]
    3: lively attentiveness [syn: alertness, sharp-sightedness,
       on the qui vive]