1) Feeling of distress or apprehension whose source is unknown.

2) Vague uneasy feeling of discomfort or dread accompanied by an autonomic response (the source often nonspecific or unknown to the individual); a feeling of apprehension caused by anticipation of danger. It is an alerting signal that warns of impending danger and enables the individual to take measures to deal with threat.

3) Apprehension or fear of impending actual or imagined danger, vulnerability, or uncertainty.