The Collaborative International Dictionary of English v.0.48:

Nutrient \Nu"tri*ent\, a. [L. nutriens, p. pr. of nutrire. See
   Nourish.]
   Nutritious; nourishing; promoting growth.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Nutrient \Nu"tri*ent\, n. n.
   1. Any substance which has nutritious qualities, i. e., which
      nourishes or promotes growth; a nutriment.
      [1913 Webster]

   2. Specifically: (Microbiology) A substance added to the
      growth medium of a microorganism to promote growth.
      [PJC]

WordNet (r) 3.0 (2006):

nutrient
    adj 1: of or providing nourishment; "good nourishing stew" [syn:
           alimentary, alimental, nourishing, nutrient,
           nutritious, nutritive]
    n 1: any substance that can be metabolized by an animal to give
         energy and build tissue [syn: food, nutrient]
    2: any substance (such as a chemical element or inorganic
       compound) that can be taken in by a green plant and used in
       organic synthesis