In the context of prioritizing requirements, risk refers to the difficulty of implementing a requirement or the likelihood that it cannot deliver its potential value. Risk-based prioritization considers uncertainties related to implementation, cost overruns, time delays, or failure to achieve desired outcomes. High-risk requirements may be prioritized lower unless mitigated effectively.
BABOK Reference - Task: Prioritize Requirements (Section 5.2):
"Risk: the chance that the requirement cannot deliver the potential value, or that the solution may not deliver the expected benefit."