Mid-continuum is a term that refers to someone, or a group of someones, who identifies as neither
a multiple nor a singletype. The definition of this term is somewhat debated by multiples and mid-continuums, since it seems difficult to draw the line between mid-continuum and multiple, since many systems who
identify as mid-cont (a common abbreviation) often share a majority of characteristics with systems who identify as multiple.
If you look at multiplicity as a dissociative disorder (which it is defined as in the DSM-IV), then mid-cont coincides with the diagnosis of DDNOS, Dissociative Disorder Not Otherwise Specified. Usually this diagnosis is given to folks who seem multiple but do not fit all the criterion for DID, Dissociative Identity Disorder, which requires "memory loss too extensive to be explained by ordinary forgetfulness". But to say that a mid-continuum is a multiple who doesn't lose time is to rashly oversimplify the experience of mid-continuums. A page on mid-cont, http://www.asarian.org/~vickis/continuum.html, describes mid-continuum as "ranging from having different 'roles' that you live out in different situations, to having an 'inner child' or 'inner children' with varying degrees of separateness, to having 'ego states', 'parts' or 'fragments' that don't seem to be whole people". Mid-continuums have also self-described as being like the lenses of a camera: they see the world with different perspectives, depending on who is out. Occasionally a mid-continuum will sometimes feel multiple and sometimes feel squashed into a single identity, leading them to believe that they are somewhere in between. Of course, many multiples have some or all of these characteristics as well. In the empowered multiple community, many people who would not be diagnosed with DID still identify as a full-fledged multiple.
So the only way to satisfactorily define mid-continuum seems to be to let self-identification rule. To me, a mid-continuum is anyone who believes they are one. The thing to stress here is that whatever label works best for the individual is best.
Sometimes a single person or people within a multiple system will identify as mid-continuum (there are also multiples within multiples, but that's another story). These cases are no different from other mid-continuums -- a person seeing themselves as slightly blurry, slightly multiple, as having different perspectives. Mid-continuums within multiple systems are sometimes closely related to age sliders.