Noun
prepared piano (plural prepared pianos)
(music) A piano that has had its sound altered by placing objects (called preparations) on or between the strings.
Amos performed solo each night on her iconic Bösendorfer piano, and on a prepared piano during "Bells for Her". Source: Internet
The scores for music for prepared piano specify the modifications, for example instructing the pianist to insert pieces of rubber, paper, metal screws, or washers in between the strings. Source: Internet