Source : Webster's Revised Unabridged Dictionary (1913)

Cambium \Cam"bi*um\, n. [LL. cambium exchange, fr. L. cambire to
   exchange. It was supposed that cambium was sap changing into
   wood.]
   1. (Bot.) A series of formative cells lying outside of the
      wood proper and inside of the inner bark. The growth of
      new wood takes place in the cambium, which is very soft.

   2. (Med.) A fancied nutritive juice, formerly supposed to
      originate in the blood, to repair losses of the system,
      and to promote its increase. --Dunglison.

Source : WordNet®

cambium
     n 1: formative one-cell-thick layer of tissue between xylem and
          phloem in most vascular plants that is responsible for
          secondary growth
     2: the inner layer of the periosteum
     [also: {cambia} (pl)]