A small naked piano, beaten with sticks and still producing a pleasing sound.
The dulcimer is a type of Zither and the type commonly referred to is a Hammer or Hammered Dulcimer. Sharing its construction with the Psaltery and Autoharp it has strings stretched over a soundboard, has no frets and plays by having many strings, each tuned to a different note. The strings are often doubled or trebled into courses. The instrument may be small enough to sit on the lap or large enough to require their own stand.
What is different about the dulcimer is that is usually has two bridges - a treble and a bass bridge. Treble strings run over the treble bridge and under the bass bridge while bass strings so the opposite. They are produced in both Diatonic and Chromatic versions and are sometimes extended with extra bridges.
Image from Marcille Wallis
Billy Bennington - The Barford Angel