The Federation Secretary of Defense is the head of the Federation Department of Defense and the individual in the Federation government who is responsible for running the UFP's defensive forces and coordinating their activities. The Secretary of Defense is a member of the Presidential Cabinet, and has significant influence upon Federation policy.
In the 2160s, this position was known as the Federation Commissioner for Defense. The inaugural Federation Defense Commissioner was Min glasch Noar, who was forced to resign in 2163 after being compromised by the Orion Syndicate. (ENT - Rise of the Federation novel: A Choice of Futures)
The Federation Secretary of Defense, obviously, is the fictional counterpart of the United States Secretary of Defense, though it is unknown whether or not the Federation Secretary of Defense is legally empowered to issue orders to the Federation Starfleet as the US Secretary of Defense is to the United States Military.