synonym (noun):

1. An incorrect or incorrectly applied scientific name, as a new name applied to a species or genus already properly named, or a specific name preoccupied by that of another species of the same genus; -- so used in the system of nomenclature (which see) in which the correct scientific names of certain natural groups (usually genera, species, and subspecies) are regarded as determined by priority.

2. One of two or more words corresponding in meaning but of different languages; a heteronym. [Rare]

pl. Synonyms (- n&ibreve;mz). [F. synonyme, L. synonyma, pl. of synonymum, Gr. synw`nymon. See Synonymous.]

One of two or more words (commonly words of the same language) which are equivalents of each other; one of two or more words which have very nearly the same signification, and therefore may often be used interchangeably. See under Synonymous. [Written also synonyme.]

All languages tend to clear themselves of synonyms as intellectual culture advances, the superfluous words being taken up and appropriated by new shades and combinations of thought evolved in the progress of society. De Quincey.

His name has thus become, throughout all civilized countries, a synonym for probity and philanthropy. Macaulay.

In popular literary acceptation, and as employed in special dictionaries of such words, synonyms are words sufficiently alike in general signification to be liable to be confounded, but yet so different in special definition as to require to be distinguished. G. P. Marsh.