synonymy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The quality of being synonymous; equivalence of meaning.
• noun Study and classification of synonyms.
• noun A list, book, or system of synonyms.
• noun Biology A chronological list of the published scientific names that have been applied to a species.

from The Century Dictionary.

• noun The quality of being synonymous, or of expressing the same meaning by different words.
• noun In rhetoric, a figure by which words of the same meaning are used to amplify a discourse.
• noun A thing of the same name.
• noun A system of synonyms; a collection of synonyms; also, the study of synonyms; the use of synonyms in expressing different shades of meaning; the discrimination of synonyms; especially, in natural history, the sifting of synonyms to determine the onyms.

from the GNU version of the Collaborative International Dictionary of English.

• noun The quality of being synonymous; sameness of meaning.
• noun A system of synonyms.
• noun (Rhet.) A figure by which synonymous words are used to amplify a discourse.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun semantics The quality of being synonymous; sameness of meaning.
• noun A list or collection of synonyms, often compared and contrasted.
• noun The study of synonyms.
• noun A system of synonyms.
• noun botany The collective synonyms (all the names referring to a particular taxon, except the correct name)
• noun botany The state of not being a correct name, of being a synonym
• noun zoology The collective synonyms (all the names referring to the same taxon, including the correct name)

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the semantic relation that holds between two words that can (in a given context) express the same meaning