contra-octave

from The Century Dictionary.

• noun In music, the 16-foot octave of the organ, the notes of which are denoted by CC, DD, etc.; on the piano, the lowest octave beginning with C, the notes of which are denoted by C₁, D₁, etc.; on other instruments, the octave corresponding to these.

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

• noun music The octave directly below the great octave