The act of putting cash into a financial institution such as a bank.
This is done as a means of security along with getting a return on one's money. The financial institution takes the money and either lends it out or invests in other securities. Under this arrangement, the bank seeks to generate a larger return than is being paid out in interest to the depositors. This difference between the two is the bank's profit.
On the balance sheet, this is listed as a liability.