Return on investment (ROI) or Rate of Return (ROR) is a straightforward tool that measures the economic return of a project or investment,
the ratio of money gained or lost on an investment relative to the amount of money invested.
The initial value of an investment does not always need to have a clearly defined monetary value; ROI may refer to "other" investments such as time invested and return for same.
Simply: What is invested and what is the return for the specific investment.
 See also