A commissioner was a member of a commission, or the leader of a governmental body. Commissions were commonly created to solve specific problems or address specific ongoing issues, and were staffed with experts in those areas, these experts were the commissioners.
In 2267, the Federation maintained some form of commission for the purpose of mediating disputes; this body dispatched assistant commissioner Nancy Hedford to Epsilon Canaris III. Her goal was to prevent the outbreak of war; her mission was cut short when she developed Sakuro's Disease and had to be recalled. (TOS: "Metamorphosis")