alert

Adjective

• mentally perceptive and responsive
  • an alert mind
    
  • alert to the problems
    
  • alive to what is going on
    
  • awake to the dangers of her situation
    
  • was now awake to the reality of his predicament
    

• quick and energetic
  • a brisk walk in the park
    
  • a lively gait
    
  • a merry chase
    
  • traveling at a rattling rate
    
  • a snappy pace
    
  • a spanking breeze
    

• engaged in or accustomed to close observation
  • caught by a couple of alert cops
    
  • alert enough to spot the opportunity when it came
    
  • constantly alert and vigilant, like a sentinel on duty
    

Noun

• an automatic signal (usually a sound) warning of danger

• a warning serves to make you more alert to danger

• condition of heightened watchfulness or preparation for action
  • bombers were put on alert during the crisis
    

Verb

• warn or arouse to a sense of danger or call to a state of preparedness
  • The empty house alarmed him
    
  • We alerted the new neighbors to the high rate of burglaries
    

Related words

• Synonyms: 
  alive(p) , awake(p) , brisk , lively , merry , rattling , snappy , spanking , zippy , watchful , alarm , warning signal , alarum , alerting , qui vive
• Antonyms: 
  unalert , unwatchful , unvigilant