What is Vapor Lock?

Legal Definition
Vapor lock is a problem that mostly affects gasoline-fueled internal combustion engines.

It occurs when the liquid fuel changes state from liquid to gas while still in the fuel delivery system. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. Restarting the engine from this state may be difficult.

The fuel can vaporize due to being heated by the engine, by the local climate or due to a lower boiling point at high altitude. In regions where higher volatility fuels are used during the winter to improve the starting of the engine, the use of "winter" fuels during the summer can cause vapor lock to occur more readily.
-- Wikipedia
Legal Definition
Situation where liquid is trapped in pipes and cannot flow as vapor has formed and is blocking the pipe.