Return on Investment is the benefit to an investor resulting from an investment of some resource.
To invest is to allocate money in the expectation of some benefit in the future.
How to calculate Return on Investment by InvestmentPropCoach
A high ROI means the investment gains compare favorably to investment cost.
Investopedia Video: How To Calculate Return On Investment (ROI) by Investopedia
As a performance measure, ROI is used to evaluate the efficiency of an investment or to compare the efficiency of a number of different investments.
In purely economic terms, it is one way of considering profits in relation to capital invested.