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

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "taxonomy":
   aerobiology, agrobiology, analysis, anatomy, animal physiology,
   anthropology, antonomasia, arrangement, assortment, astrobiology,
   bacteriology, binomial nomenclature, biochemics, biochemistry,
   biochemy, bioecology, biological science, biology, biometrics,
   biometry, bionics, bionomics, biophysics, biosystematics,
   biosystematy, botany, cataloging, categorization, cell physiology,
   class, classification, codification, comparative anatomy,
   conchology, cryobiology, culling, cybernetics, cytology, division,
   ecology, electrobiology, embryology, entomology, enzymology,
   ethnobiology, ethology, exobiology, family, filing, genetics,
   genus, glossology, gnotobiotics, gradation, grading, grouping,
   helminthology, herpetology, ichthyology, indexing, kingdom,
   life science, malacology, mammalogy, microbiology,
   molecular biology, nomenclature, onomastics, onomatology, order,
   orismology, ornithology, pharmacology, phylum, physiology,
   pigeonholing, place-names, place-naming, placement, polyonymy,
   protozoology, radiobiology, ranging, ranking, rating, screening,
   selection, sifting, sorting, sorting out, species, stratification,
   subdivision, subordination, systematics, tabulation, taxidermy,
   terminology, toponymy, triage, trinomialism, typology, virology,
   xenobiology, zoogeography, zoography, zoology, zoonomy,
   zoopathology, zoophysics, zootaxy, zootomy