Return on Investment [ROI] is what you get back from the investment of money or time or effort that you put into some project. The "project" can be a marketing campaign, your career, the business you own or where you work, a personal or business relationship, or whatever project into which you sink significant time, effort or money in hopes of getting something valuable back from it.
Celebrating 25 Years of Creating an Island of Stability In the Sea of Online Confusion