HumanitiesWeb.org - Glossary definition: Una Corda Pedal
Una Corda Pedal (from the Italian, meaning 'one string'.)
This is the pedal on the piano that causes the hammer to hit one string and not all three. It enables a pianist to play more quietly, as well as changing the tone. Also known as the damper pedal, it is often referred to as the 'soft' pedal.