quick to notice any unusual and potentially dangerous or difficult circumstances; vigilant
an alert police officer discovered a lorry full of explosive
schools need to be constantly alert to this problem
the state of being watchful for possible danger
security forces are on the alert for an upsurge in violence
an announcement or signal warning of danger
an alert sounded and all the fighters took off
a flood alert has been issued
a bomb alert
warn (someone) of a danger or problem
he alerted people to the dangers of smoking
police were alerted after three men drove away without paying