A string is the
vibrating element which is the source of vibration in
string instruments, such as the
guitar,
harp,
piano, and members of the
violin family. They are lengths of a flexible material kept under
tension so that they may freely vibrate. Strings may be "plain" (consisting only of a single material, like steel, nylon, or gut) or they may have a core of one material with an overwinding of other materials to add mass so the string sounds the desired pitch, while keeping the string thin and flexible enough to play. Such strings are called "wound strings."
See more at Wikipedia.org...