(1):

(n.) 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):

(n.) 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.

(3):

(n.) One of two or more words corresponding in meaning but of different languages; a heteronym.