From the article:

Return on Investment… the holy grail of IT.

Simply put, ROI is defined as the “ratio of money gained or lost on an investment relative to the amount of money invested”. One formula used to determine ROI is “net income plus interest divided by the book value of assets equals Return On Investment.“

In real terms, when you invest in a technology for your business, it’s about more than that. IT-related ROI often needs to provide cost savings, rather than generate revenue. In the case of virtualization for consolidation, this is often a simple calculation made difficult by many variables.

More here.