A certificate of deposit (CD) that allows its holder, and contrary to most types of CDs, to deposit more money into the CD account during the lifespan of the certificate. The account holder of a vanilla CD can only make the initial opening deposit, as depositee banks do not allow any additional deposits into the same account. However, an add-on CD is very much similar to a savings account, where additions are possible, virtually anytime

A given bank issuing such certificates would clearly define the number of additional deposits a depositor can make over the course of the CD term. In practice, this number varies from an institution to another.