Definition
In mining, ROI typically refers to the payback period: the number of days or months before cumulative mining profits (revenue minus electricity) equal the hardware purchase price. After the payback period, all subsequent profit is net gain.
ROI calculations should account for difficulty increases over time, potential Bitcoin price changes, and hardware resale value. Shorter ROI periods are better, but even long-ROI miners can be worthwhile if the operator is accumulating Bitcoin for long-term holding.
In Simple Terms
The time for mining profits to pay back the hardware cost. Shorter payback periods are better.
Return on Investment. The time or ratio measuring how long it takes for mining revenue to pay back the initial hardware investment.