What is a sudden strong feeling of fear that prevents reasonable thought and action called?

Panic is a sudden sensation of fear, which is so strong as to dominate or prevent reason and logical thinking, replacing it with overwhelming feelings of anxiety and frantic agitation consistent with an animalistic fight-or-flight reaction. Panic may occur singularly in individuals or manifest suddenly in large groups as mass panic.

The word "panic" derives from antiquity and is a tribute to the ancient god Pan. One of the many gods in the mythology of ancient Greece, Pan was the god of shepherds and of woods and pastures. The Greeks believed that he often wandered peacefully through the woods, playing a pipe, but when accidentally awakened from his noontime nap he could give a great shout that would cause flocks to stampede. From this aspect of Pan's nature Greek authors derived the word "panikos", “sudden fear,” the ultimate source of the English word: "panic".