vigilant

adj.

[1]

watchful , argus-eyed , open-eyed , alert , sharp , wakeful , observant , guarded , circumspect , attentive , wakeful , cautious , careful , wary , chary , on one's guard , on the alert , on the lookout , eagle-eyed , hawk-eyed , Argus-eyed , on the qui vive , on one's toes , with one's eyes open , (Colloq) with one's eyes skinned/peeled