vig·i·lant 
(vĭjə-lənt)
(vĭjə-lənt)adj.
On the alert, as for danger or error; watchful. See Synonyms at careful.
[Middle English, from Old French, from Latin vigilāns, vigilant-, present participle of vigilāre, to be watchful; see VIGILANTE.]
vigi·lant·ly adv.