NOUN (3)

1. the passage of gases through fine tubes because of differences in pressure or temperature;

2. the process of giving off or exhaling water vapor through the skin or mucous membranes;

3. the emission of water vapor from the leaves of plants;

The Collaborative International Dictionary of English v.0.48:

Transpiration \Tran`spi*ra"tion\, n. [F. transpiration.]
   1. (Physiol.) The act or process of transpiring or excreting
      in the form of vapor; exhalation, as through the skin or
      other membranes of the body; as, pulmonary transpiration,
      or the excretion of aqueous vapor from the lungs.
      Perspiration is a form of transpiration. --Cudworth.
      [1913 Webster]

   2. (bot.) The evaporation of water, or exhalation of aqueous
      vapor, from cells and masses of tissue.
      [1913 Webster]

   3. (Physics) The passing of gases through fine tubes, porous
      substances, or the like; as, transpiration through
      membranes.
      [1913 Webster]

WordNet (r) 3.0 (2006):

transpiration
    n 1: the passage of gases through fine tubes because of
         differences in pressure or temperature
    2: the process of giving off or exhaling water vapor through the
       skin or mucous membranes
    3: the emission of water vapor from the leaves of plants