How to … Tell if a name is legitimate or illegitimate
A legitimate name is a validly published name that is not illegitimate.
A validly published name is illegitimate if:
It is a later homonym (Art. 53). There is an earlier, validly published name with the same spelling and a different type (the two names are then homonyms). Note that rank-denoting terms are ignored when determining homonymy: for example, the names of a variety and subspecies under the same species, or the names of a section and subgenus under the same genus, will be homonyms if their epithets are the same but their types are different.
It was nomenclaturally superfluous at the time it was published (Art. 52). A name is superfluous if there is an earlier name that the publishing author should have used (but was perhaps unaware of). This will be the case if the author included the type of the earlier name in the taxon to which the new name was applied, or if the earlier name was cited as a synonym of the new name.
Note that an illegitimate name can be made legitimate through conservation. Making a name legitimate may be important if, for example, it is widely used and changing it (to the earliest legitimate name) would be very disadvantageous for users. See How to conserve a name.
For more information, see Chapter 2 Basic concepts and terms in The Code Decoded.