The Collaborative International Dictionary of English v.0.48:

Apothem \Ap"o*them\, n. [Gr. 'apo` + qe`ma that which is placed,
   tiqe`nai to place.]
   1. (Math.) The perpendicular from the center to one of the
      sides of a regular polygon.
      [1913 Webster]

   2. A deposit formed in a liquid extract of a vegetable
      substance by exposure to the air.
      [1913 Webster]