A certificate of deposit (CD) that allows its holder, and contrary to most types of certificates of deposit, to deposit more money into the certificate of deposit account during the lifespan of the certificate. The account holder of a vanilla certificate of deposit can only make the initial opening deposit, as depositee banks do not allow any additional deposits into the same account. However, an add-on certificate of deposit 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 certificate of deposit term. In practice, this number varies from an institution to another.