A three or four letter abbreviation used to identify a security whether on the floor, a TV screen, or a newspaper page. Ticker symbols are part of the lore of Wall Street. They were originally developed in the 1800s by telegraph operators to save bandwidth. One-letter symbols were therefore assigned to the most active stocks. Railroads were the dominant issues at the time, so they retain a majority of the one-letter designations.
Ticker symbols today are assigned on a first-come, first-served basis. Each marketplace -- the NYSE, the American Stock Exchange, and others -- allocates symbols for companies within its purview, working closely to avoid duplication. A symbol used for one company cannot be used for any other, even in a different marketplace.