The Psaltery is a harp on a box. Any box with different lengths of strings on is a Psaltery. The box could be many different shapes, my one is a long triangle.
The Ancient Greek Psaltery is a small harp, called a psalterian. The box on strings version is seen in carvings, illustrations and paintings from around the 12th century.
In the 19th Century there was a renewed interest in the Psaltery. It was renamed Zither, Autoharp, and Bowed Psaltery. Which to me sounds wonderful, but unfortunately there is no evidence that the psaltery was ever bowed in the Medieval age. But there is very little evidence that anything was done in the Medieval age so hopefully there were bowed psalteries as well. If you used a small stick to make sounds on the instrument instead of plucking it was called a Dulcimer.