fear resulting from the awareness of danger
"The sudden crackle of dry twigs sent a sharp alarm through her as she realized she was being watched in the dark woods."
a device that signals the occurrence of some undesirable event
"The fire alarm rang loudly when the smoke detector sensed heat in the kitchen."
an automatic signal (usually a sound) warning of danger
"The loud fire alarm suddenly echoed through the hallway, forcing everyone to evacuate immediately."
a clock that wakes a sleeper at some preset time
"She hit snooze on her alarm three times before finally getting out of bed to start her day."
A summons to arms, as on the approach of an enemy.
"The distant sound of the alarm signaled that our village was under immediate attack by the invading forces."
In plain English: An alarm is a loud noise that warns you of danger so you can react quickly.
"The loud fire alarm woke everyone up early in the morning."
Usage: In everyday usage, an alarm is a signal or device that warns of danger or trouble, such as a fire alarm or a panic in a crowd. While it can historically refer to a call to arms against an enemy, people now primarily use it for any immediate warning of a threat.
To call to arms for defense
"When the border was breached, the general sounded the alarm to rally all available troops for an immediate counterattack."
In plain English: To alarm someone is to make them feel scared or worried about something bad that might happen.
"The sudden noise alarmed the sleeping child."
Usage: Use the verb alarm to mean frightening or causing someone to feel sudden worry, not to summon them for battle. For example, the loud noise alarmed the sleeping children rather than calling them to defend their home.
The word "alarm" comes from a military shout in Old Italian meaning "to arms!" which was originally used to call soldiers to their weapons. It traveled into English through Middle French and Middle English while keeping its original sense of an urgent warning.