It stands for money market deposit account; a category of money market vehicles (MMV) that represents a savings account may provide its holder with certain other privileges such as debit card and check-writing. It is an interest-paying account at a bank or similar entity (e.g., thrifts, credit unions) that allows uncapped withdrawals (e.g., through ATM), but with a limited number of purchases/ transfers within a certain period of time (e.g., 6 months).

A money market deposit account is a sort of risk-free deposit accounts that carries the earning potential of a savings account and the convenience of a checking account. It is a good solution for market participants who seek the higher interest rate of a savings account but want certain level of  flexibility as to have access to the funds (e.g., by means of writing checks). Certain banks require minimum balances for a depositor to start earning the higher interest rate, but many online providers do not require that.

Money market deposit accounts differ from money market funds (MMFs) in the sense that the latter (also known as money market mutual funds) are investments, not deposit accounts. At the backend, investments are run through a fund structure, and do not entitle share owners access to their funds through cards or checking facility.