Allow a role to edit a specific taxonomy list of terms
Taxonomy Access Control provides this functionality.  Workbench does as well, though it is slightly more complex and from my limited experience performs better when only a single term is used per node to control access.  Workbench (access + moderation) includes a nice set of tools for content moderation and workflow not found in Taxonomy Access Control. Both are worth a look before you decide for your use case.