This is an unfortunate term used in psychology, often abbreviated as "host" and intended to group together between systems the one resident that asks for help, with the assumption being that this person is the normal front of the system and is mainly unaware of their multiplicity. This is the person most psychologists are "stuck" working with most of the time, until other residents feel comfortable enough (or desperate enough) to surface. Often the "host" in psychology's terms is the person who maintains the majority of daily functioning. (One of the many problems with this is that psychologists often assume the first person who presents for therapy is the 'host' and work from there; or, when faced with a system which has no 'primary' frontrunner and where several people share time equally, will try to arbitrarily peg one as being the host.)
Most of the time, in other words, psychologists are stuck working with someone who doesn't know about anyone else in the system, who is in deep denial of their multiplicity, and who deals with daily stress at work, home, etc. This person may believe, for example, that their problem is depression and often if the multiplicity becomes apparent, the multiplicity is blamed as being the root of the problem, rather than the depression, etc.
A similar (mis)understanding and misconception of the circumstance can be felt by loved ones of those who are discovering their multiple state.
See also other sections of this manual that deal with multiplicity not being the real problem... :P