ROI is the acronym that stands for Return-On-Investment
A very straightforward and simple concept; ROI / Return on Investment directly means what it says; what an investor can be expecting to make or has made off of an investment.
example:
Harry bought some Litecoins in late 2015 for ~$3.50 and sold them all at ~$150; his ROI is 4185.71%. Today Harry is his flying lamb-ercedes to the moon and back for fun.