How much profit you made from money you spent. Compares what you gained against what it cost.
ROI is the ultimate reality check for your marketing. If you spend $1,000 on ads and get $800 in revenue, you are paying for the privilege of working. Every marketing activity—from a blog post to an ad campaign—should have a projected ROI. If it doesn't pay for itself over time, kill it and put that energy somewhere else.