ROI is short for "return on investment." This is a poker term that is used to measure the profitability of one's play in poker tournaments. ROI is calculated as (100 * total profit/total buy-in). Thus, if one received a prize of $150 in a tournament with a buy-in of $100+$9, one's ROI is calculated as 100 * $41/$109= 37.61%. ROI is used for tournaments though because tournaments always have a fixed buy-in in relation to the prize pool. ROI is not useful for ring game play. Some people keep track of their ROI for ring games based on their buy-in and how much they cash out. This is silly. Hourly rate is the measuring stick for ring game play. For example, if one wins $10 at $.50-$1 limit over 5 hours, that person's hourly rate is $2/hour. It will not matter if that person buys in for $100 at that game or $1 million, he or she made $2/hour. However, that person's ROI drastically changes. At a buy-in of $100, that person's ROI is 10%. At a buy-in of a million, that person's ROI is .001%. You can keep track of your own ROI for free at www.checkyourbets.com.
See also Hourly Rate.
(Submitted by TwoGun)