Stringed instruments are classified by how energy enters their strings

Evidence

There are three main methods by which energy is provided to a stringed instrument. The strings are either “plucked,” “bowed” or “struck.” Instruments which are usually plucked or bowed include those in the violin family; instru­ ments whose strings are generally only plucked include the guitar, lute, and harpsichord; and the piano is an instrument whose strings are struck. — David Howard, Jamie Angus, coustics and Psychoacoustics, 1100