A cash flow is a real or virtual movement of money:

a cash flow in its narrow sense is a payment (in a currency), especially from one central bank account to another; the term 'cash flow' is mostly used to describe payments that are expected to happen in the future, are thus uncertain and therefore need to be forecast with cash flows;
a cash flow is determined by its time t, nominal amount N, currency CCY and account A; symbolically CF = CF(t,N,CCY,A).
it is however popular to use cash flow in a less specified sense describing (symbolic) payments into or out of a business, project, or financial product.Cash flows are narrowly interconnected with the concepts of value, interest rate and liquidity.
A cash flow that shall happen on a future day tN can be transformed into a cash flow of the same value in t0.