What is a 'tank slapper' or handlebar wobble (bamboleo / shimmy) and what is the proper action if it occurs?

1. A celebration gesture by motocross riders
2. A violent oscillation of the handlebars from lock to lock caused by front wheel misalignment or landing; the rider should maintain a firm but relaxed grip, avoid fighting the oscillation stiffly, close the throttle gradually, and avoid slamming the front brake
3. The fuel tank overflowing with gasoline
4. An automated feature of the cruise control