What is it called when one light goes out and the rest are still lit up?

This may apply to lights that are wired in parallelacross a voltage source. In a parallel circuit, like the outlets in a home, the same voltage is applied to each thing that is plugged into the outlet. That way all the appliances can be designed for a constant operating voltage and if one fails, the rest continue to operate uaffectedly. Picture a railroad track as the two sides of the electrical bus. Everything hooked up is attached "across" the rails, and works on the voltage that is on the rails and operates independently of other things across the rails. When we talk about a power bus in electronics, we sometimes refer to the "rails" of the bus - the two sides of the supply.