A value of discount rate which nullifies the project's NPV :
\mbox{NPV} = \sum_{i=0}^n \frac{R_i}{(1+\mbox{IRR})^i} = 0 |
The usual practise is to give preferences to the investment projects with higher IRR and make a direct comparison of IRR against the Weighted Average Cost of Capital (WACC).
See also
Economics / Investment / Financial Investment / Cash Discount