sound post

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A section of dowel that spans the gap between the sounding board and the back plate of a stringed instrument such as a violin or cello, providing structural support and improving the instrument's tone quality.

from The Century Dictionary.

• noun In musical instruments of the viol class, a small cylindrical wooden prop or pillar which is inserted between the belly and the back, nearly under the treble foot of the bridge.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun music A dowel located in the interior of an instrument of the violin family, sandwiched between the sound board and the back plate (but not glued to them), whose primary purpose is to counter the pressure of the tensed strings on the sound board (through the bridge), and whose beneficial side effect is to transmit sound from the sound board to the back plate, aiding to produce resonance.