A money market security is a type of investment that allows individuals, companies, and governments to borrow or lend money for a short period of time. 

Money market securities are typically considered to be low-risk investments that provide relatively stable returns.

Money market securities are issued by a variety of organizations, including corporations, banks, and government entities. 

They are typically traded in large denominations, but smaller investors can gain access to these securities through money market funds. 

There are several types of money market securities, each with its own unique characteristics. 

One common type is the Treasury bill (T-bill), which is issued by the U.S. government. 

T-bills have a maturity of one year or less and are considered to be very safe investments.

Another type of money market security is commercial paper, which is issued by corporations to raise short-term funds.