Something, (or perhaps
nothing) that
can't, (or
can) be
experienced.
Observable things fall into two categories :
Things which can be experienced by an
unaided human being, and those which can be experienced only by the
aid of, or measurement by, an
external device.
Thus, the
unobservable can not be experienced by a human, or an external device. Some people believe that
unobservable things do not exist. Belief in the unobservable usually requires
faith.