A Certificate of Deposit. A kind of simple interest bank account adjacent in form factor to bonds. In simple terms, you give the bank an amount of money for a set amount of time, say a year, during which you can't withdraw the money without paying a penalty. When the term is finished the bank returns the money plus an amount of simple interest. They often bear higher interest rates than savings or money market accounts and are FDIC insured.

CDs nuts!

CD is just an acronym for the Compact Disc media format.

You can get one at a bank. It is short for certificate of deposit. It's basically a high interest savings account that is difficult to access until it matures. They are at or above 4% now at some banks, so it is a relatively safe investment that is slightly higher than inflation.