A medieval seneschal was the manager or steward of a lord or lady's household. They were responsible for the staff within the estate and making sure everything ran smoothly.
In the SCA, the seneschal is the legal representative of a branch. Seneschals sign all contracts, make sure that things are done pursuant to the restrictions of all modern and SCA laws, work with the Chatelaine to handle all public relations, and normally conduct the business meetings. The Seneschal coordinates the branch officers.