Noah Webster's New International Dictionary of the English Language

1. (n.) A plant of the genus Panicum; panic grass; also, the edible grain of some species of panic grass.

2. (a.) Extreme or sudden and causeless; unreasonable; -- said of fear or fright; as, panic fear, terror, alarm.

3. (n.) A sudden, overpowering fright; esp., a sudden and groundless fright; terror inspired by a trifling cause or a misapprehension of danger; as, the troops were seized with a panic; they fled in a panic.

4. (a.) By extension: A sudden widespread fright or apprehension concerning financial affairs.