A steward was responsible for the management of a household, like a seneschal. The scope of their powers and duties varied depending on the size of the household. Some were large enough that the steward managed the land and finances for the entire estate.
In Heraldry, stewards are represented by the Cornucopia, the Horn of Plenty.
Stewards in the SCA
Steward is sometimes used within the SCA as an alternative title to autocrat, especially when one wishes to be more period. A steward can also refer to someone who manages the service at feast - announcing courses and directing servers.