alert >> /əˈlərt/
Definition: [transitive verb] Warn (someone) of a danger, threat, or problem, typically with the intention of having it avoided or dealt with.
Example: he alerted people to the dangers of smoking
Definition: [noun] The state of being watchful for possible danger.
Example: security forces were placed on alert
Definition: [adjective] Quick to notice any unusual and potentially dangerous or difficult circumstances; vigilant.
Example: an alert police officer discovered a truck full of explosives