■ adjective quick to notice and respond to potential danger or problems.
↘intellectually active.
■ noun
1》 the state of being ~: everyone should be on the ~ for terrorists.
2》 a warning of danger.
3》 a prompt or similar signal on an electronic device.
■ verb warn of a danger or problem.
Derivatives
~ly adverb
~ness noun
Origin
C16 (orig. in military use): from Fr. ~e , from Ital. all' erta 'to the watchtower'.