A deposit is the act of placing cash (or cash equivalent) with some entity, most commonly with a financial institution, such as a bank.
The deposit is a credit for the party (individual or organization) who placed it, and it may be taken back (withdrawn) in accordance with the terms agreed at time of deposit, transferred to some other party, or used for a purchase at a later date.
Deposits are usually the main source of funding for banks.
This the foundation of fractional-reserve banking, since the bank can lend out the money that it owns while owing an obligation to the depositor.
A special deposit is one made under an agreement to hold the deposit separately from the bank's assets, so that the same assets can be returned.