1 a
: a grammatical construction in which two usually adjacent nouns having the same referent stand in the same syntactical relation to the rest of a sentence
[M-W's Col. Dic.]

What is a syntactical relation?
The two nouns (referent and appositive) have the same syntax within the sentence. For example:

My brother, the lawyer, is an ass. My brother and the lawyer are both singular nouns which have the same relationship to the verb (My brother is; the lawyer is).
Thank you, Mr Micawber.