A hospitaller was someone associated with aiding the sick in a hospital (i.e. giving them hospice). This was eventually extended to be used for any of the Knights of St John, who set up a hospital for pilgrims in Jerusalem during the Crusades.
Hospitallers in the SCA
A Hospitaller, sometimes called chatelaine or castellan depending on the Kingdom, is responsible for looking after the needs of new members to the SCA. Though, you will find most people look after newbies anyway, but it's specifically their job to assist them with garb and feasting gear, if so required. They may also be in charge of storing the regalia for their group.
NOTE: There is an old tradition in the SCA of adding the letter “e” after some offices to denote they are being held by a female, e.g. Chatelain or Chatelaine, Seneschal or Seneschale. This is up to the Kingdom and Local Groups if they want to do this.
What a Hospitaller/Chatelaine/Castellan does varies from group to group. In a perfect world, the Chatelaine is responsible for providing an introduction to the Current Middle Ages to new or prospective members. They should be available to advise newcomers of the conventions of the Society, their Kingdom and group; and for encouraging participation in the group. The Chatelaine can provide access to educational information such as handouts, booklets and other materials. In all of these activities, the Chatelaine's main charge is to represent the SCA accurately and favorably, and to encourage all members to do likewise.
The Hospitaller/Chatelaine/Castellan is a very important office within a group. Without new members, the SCA would stagnate. Without the influx of new energy and new ideas, the same people would go on doing the same things year after year until they would burn out or drop out. Without positive public exposure, the Society would suffer from stereotyping, rumors and have a hard time interacting with other groups and individuals.
Whenever possible, the Hospitaller/Chatelaine/Castellan should be someone who has been a member of the SCA, and the local group, for a number of years and is someone who knows and understands the group’s dynamics. They should have a pleasant personality and get along well with others. Outgoing enough to meet and talk to new people, but understanding enough to give some people the space they may want. The Chatelaine should be someone whom the Seneschal feels could represent the group well to others.
In many groups, the Hospitaller/Chatelaine/Castellan can also be responsible for group demonstrations, (or "demos" as they're commonly called), the Gold Key Office (which maintains garb and feast gear) and/or for Publicity.