The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cambium
    n 1: a formative one-cell layer of tissue between xylem and
         phloem in most vascular plants that is responsible for
         secondary growth
    2: the inner layer of the periosteum