Medium of exchange
Medium of exchange is one of the functions of money. When people talk about cashflow they are usually talking specifically about the utility of this function, which is to have to carry only one kind of thing around to get all other things. If you "put it in a bank" you accept some standard of deferred payment - you have to retrieve the money before you can use it for exchange, or use some trust based instrument like a cheque to do so or use the recently introduced real time verified and debited electorinic payment services like Visa Electron (physical), Debit MasterCard (physical) or Verified by Visa (network based).
See w:medium of exchange for a detailed discussion of theory.