A ~ is a person who stands in front of an orchestra or choir and directs its performance.
On a bus, the ~ is the person whose job is to sell tickets to the passengers.
On a train, a ~ is a person whose job is to travel on the train in order to help passengers and check tickets. (AM; in BRIT, use guard )
A ~ is a substance that heat or electricity can pass through or along.
see also lightning ~ , semi~