What is the definition or description of: foot drop?

Foot drop: The inability to raise (extend) your foot at your ankle usually due to a problem associated with the common peroneal nerve (and also lower spine problems). In severe cases, the patient is completely unable to raise the foot. In milder cases, the ability to walk on your heels is impossible.

See below: It is a condition where the nerve and muscle are injured and will not allow control of the foot to be a smooth motion when walking but instead allows the foot to slap down or drop when walking.

Foot slapping when : Walking caused by weakness of anterior ankle muscle groups. Weakness or paralysis of these muscles causes feet to trip or drag when walking. An posterior splint with spring hinges may be useful to help support the foot.