The ratio of profits (or losses) to the amount invested.
Example: $200 profit / $1000 investment = 20% ROI
In theory, ROI is a very simple concept and formula. However, it gets more complicated when trying to reliable track and match outcomes vs. costs in a multi-channel environment.
Other potential enemies of ROI in the real world include corporate politics (a department wanting to put ROI killing content on the company home page) and individual egos (a business owner wanting to appear #1 for a favorite search phrase with little regard for the cost or conversion rate).
see also: Digital Marketing Tools
rate of return, rate of profit