What is a 'tank slapper' (speed wobble) on a motorcycle and what is the proper rider reaction?

1. A method of tapping the fuel tank to measure fuel level
2. A violent, uncontrolled oscillation of the front handlebars back and forth; keep a relaxed grip, do not fight the bars or brake hard, and ease off throttle to allow chassis geometry to self-stabilize
3. A passenger tapping the rider on the back
4. The engine vibrating when idling in neutral