the fear resulting from the sudden sensing of danger

a signal, e.g. a loud noise or flashing light, that warns or alerts, or an automatic device that produces such a signal

a burglar alarm

= alarm clock

to strike (somebody) with fear, apprehension, cause great fear or anxiety in

Don't be alarmed, it's quite safe!

to warn or alert (somebody)

to fit or protect (something) with an automatic alarm