NOUN (3)

1. a classification of organisms into groups based on similarities of structure or origin etc;

2. (biology) study of the general principles of scientific classification;

3. practice of classifying plants and animals according to their presumed natural relationships;

The Collaborative International Dictionary of English v.0.48:

Taxonomy \Tax*on"o*my\ (t[a^]ks*[o^]n"[-o]*m[y^]), n. [Gr.
   ta`xis an arrangement, order + no`mos a law.]
   1. That division of the natural sciences which treats of the
      classification of animals and plants, primarily by
      consideration of their natural relationships with respect
      to their structure or genetic origin; the laws or
      principles of classification; systematics.
      [1913 Webster +PJC]

   2. A systematic arrangement of objects or concepts showing
      the relations between them, especially one including a
      hierarchical arrangement of types in which categories of
      objects are classified as subtypes of more abstract
      categories, starting from one or a small number of top
      categories, and descending to more specific types through
      an arbitrary number of levels. An ontology usually
      contains a taxonomy as one of the important principles of
      organization.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

taxonomy
    n 1: a classification of organisms into groups based on
         similarities of structure or origin etc
    2: (biology) study of the general principles of scientific
       classification
    3: practice of classifying plants and animals according to their
       presumed natural relationships