Noun

1. Fear resulting from the awareness of danger
2. A device that signals the occurrence of some undesirable event
3. An automatic signal (usually a sound) warning of danger
4. A clock that wakes a sleeper at some preset time

Verb

1. Fill with apprehension or alarm; cause to be unpleasantly surprised
2. Warn or arouse to a sense of danger or call to a state of preparedness